Handle multiple files localtion lists with gcc handler (#1034)

Handle multiple files in the GCC handler.

test/handler/test_vint_handler.vader

@@ -10,12 +10,14 @@ Execute(The vint handler should parse error messages correctly):
   \   {
   \     'lnum': 1,
   \     'col': 1,
+  \     'filename': 'gcc.vim',
   \     'text': 'Use scriptencoding when multibyte char exists (see :help :script encoding)',
   \     'type': 'W',
   \   },
   \   {
   \     'lnum': 3,
   \     'col': 17,
+  \     'filename': 'gcc.vim',
   \     'end_col': 18,
   \     'text': 'Use robust operators ''==#'' or ''==?'' instead of ''=='' (see Google VimScript Style Guide (Matching))',
   \     'type': 'W',
@@ -23,6 +25,7 @@ Execute(The vint handler should parse error messages correctly):
   \   {
   \     'lnum': 3,
   \     'col': 8,
+  \     'filename': 'gcc.vim',
   \     'end_col': 15,
   \     'text': 'Make the scope explicit like ''l:filename'' (see Anti-pattern of vimrc (Scope of identifier))',
   \     'type': 'W',
@@ -30,6 +33,7 @@ Execute(The vint handler should parse error messages correctly):
   \   {
   \     'lnum': 7,
   \     'col': 8,
+  \     'filename': 'gcc.vim',
   \     'end_col': 15,
   \     'text': 'Undefined variable: filename (see :help E738)',
   \     'type': 'W',
@@ -37,6 +41,7 @@ Execute(The vint handler should parse error messages correctly):
   \   {
   \     'lnum': 8,
   \     'col': 11,
+  \     'filename': 'gcc.vim',
   \     'end_col': 16,
   \     'text': 'E128: Function name must start with a capital or contain a colon: foobar (see ynkdir/vim-vimlparser)',
   \     'type': 'E',
@@ -44,6 +49,7 @@ Execute(The vint handler should parse error messages correctly):
   \   {
   \     'lnum': 9,
   \     'col': 12,
+  \     'filename': 'gcc.vim',
   \     'end_col': 13,
   \     'text': 'Use robust operators ''=~#'' or ''=~?'' instead of ''=~'' (see Google VimScript Style Guide (Matching))',
   \     'type': 'W',