Added clang-tidy for C, too.

A limited number of clang-tidy checks can be used with C, too. I pretty much
copied and refactored the C++ clang-tidy linter, and added some documentation
about C-compatible checks.

doc/ale-c.txt

@@ -2,6 +2,37 @@
 ALE C Integration                                               *ale-c-options*
 
 
+===============================================================================
+Global Options
+
+g:ale_c_build_dir_names                               *g:ale_c_build_dir_names*
+                                                      *b:ale_c_build_dir_names*
+
+  Type: |List|
+  Default: `['build', 'bin']`
+
+  A list of directory names to be used when searching upwards from cpp
+  files to discover compilation databases with. For directory named `'foo'`,
+  ALE will search for `'foo/compile_commands.json'` in all directories on and above
+  the directory containing the cpp file to find path to compilation database.
+  This feature is useful for the clang tools wrapped around LibTooling (namely
+  here, clang-tidy)
+
+
+g:ale_c_build_dir                                           *g:ale_c_build_dir*
+                                                            *b:ale_c_build_dir*
+
+  Type: |String|
+  Default: `''`
+
+  A path to the directory containing the `compile_commands.json` file to use
+  with c-family linters. Usually setting this option to a non-empty string
+  will override the |g:ale_c_build_dir_names| option to impose a compilation
+  database (it can be useful if multiple builds are in multiple build
+  subdirectories in the project tree).
+  This feature is also most useful for the clang tools linters, wrapped
+  aroung LibTooling (namely clang-tidy here)
+
 ===============================================================================
 clang                                                             *ale-c-clang*
 
@@ -21,6 +52,104 @@ g:ale_c_clang_options                                   *g:ale_c_clang_options*
   This variable can be changed to modify flags given to clang.
 
 
+===============================================================================
+clangtidy                                                   *ale-c-clangtidy*
+
+`clang-tidy` will be run only when files are saved to disk, so that
+`compile_commands.json` files can be used. It is recommended to use this
+linter in combination with `compile_commands.json` files.
+Therefore, `clang-tidy` linter reads the options |g:ale_c_build_dir| and
+|g:ale_c_build_dir_names|. Also, setting |g:ale_c_build_dir| actually
+overrides |g:ale_c_build_dir_names|.
+
+
+g:ale_c_clangtidy_checks                             *g:ale_c_clangtidy_checks*
+                                                     *b:ale_c_clangtidy_checks*
+  Type: |List|
+  Default: `['*']`
+
+  The checks to enable for clang-tidy with the `-checks` argument.
+
+  All options will be joined with commas, and escaped appropriately for
+  the shell. The `-checks` flag can be removed entirely by setting this
+  option to an empty List.
+
+  Not all of clangtidy checks are applicable for C. You should consult the
+  clang documentation for an up-to-date list of compatible checks.
+
+  Here's an (incomplete) list of C-compatible checks, for clang-tidy version 6:
+    bugprone-suspicious-memset-usage
+    cert-env33-c
+    cert-err34-c
+    cert-flp30-c
+    google-runtime-int
+    llvm-header-guard
+    llvm-include-order
+    misc-argument-comment
+    misc-assert-side-effect
+    misc-bool-pointer-implicit-conversion
+    misc-definitions-in-headers
+    misc-incorrect-roundings
+    misc-macro-parentheses
+    misc-macro-repeated-side-effects
+    misc-misplaced-const
+    misc-misplaced-widening-cast
+    misc-multiple-statement-macro
+    misc-non-copyable-objects
+    misc-redundant-expression
+    misc-sizeof-expression
+    misc-static-assert
+    misc-string-literal-with-embedded-nul
+    misc-suspicious-enum-usage
+    misc-suspicious-missing-comma
+    misc-suspicious-semicolon
+    misc-suspicious-string-compare
+    misc-swapped-arguments
+    modernize-redundant-void-arg
+    modernize-use-bool-literals
+    performance-type-promotion-in-math-fn
+    readability-braces-around-statements
+    readability-else-after-return
+    readability-function-size
+    readability-identifier-naming
+    readability-implicit-bool-cast
+    readability-inconsistent-declaration-parameter-name
+    readability-misleading-indentation
+    readability-misplaced-array-index
+    readability-named-parameter
+    readability-non-const-parameter
+    readability-redundant-control-flow
+    readability-redundant-declaration
+    readability-redundant-function-ptr-dereference
+    readability-simplify-boolean-expr
+
+
+g:ale_c_clangtidy_executable                 *g:ale_c_clangtidy_executable*
+                                               *b:ale_c_clangtidy_executable*
+  Type: |String|
+  Default: `'clang-tidy'`
+
+  This variable can be changed to use a different executable for clangtidy.
+
+
+g:ale_c_clangtidy_options                       *g:ale_c_clangtidy_options*
+                                                  *b:ale_c_clangtidy_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to modify flags given to clang-tidy.
+
+  - Setting this variable to a non-empty string,
+  - and working in a buffer where no compilation database is found using
+    |g:ale_c_build_dir_names| or |g:ale_c_build_dir|,
+  will cause the `--` argument to be passed to `clang-tidy`, which will mean
+  that detection of `compile_commands.json` files for compile command
+  databases will be disabled.
+  Only set this option if you want to control compiler flags
+  entirely manually, and no `compile_commands.json` file is in one
+  of the |g:ale_c_build_dir_names| directories of the project tree.
+
+
 ===============================================================================
 cppcheck                                                       *ale-c-cppcheck*
 

doc/ale.txt

@@ -16,6 +16,7 @@ CONTENTS                                                         *ale-contents*
       gcc.................................|ale-asm-gcc|
     c.....................................|ale-c-options|
       clang...............................|ale-c-clang|
+      clangtidy...........................|ale-c-clangtidy|
       cppcheck............................|ale-c-cppcheck|
       gcc.................................|ale-c-gcc|
     chef..................................|ale-chef-options|