Fix C flag parsing and tests on Windows

autoload/ale/c.vim

@@ -356,9 +356,27 @@ function! ale#c#ParseCompileCommandsFlags(buffer, file_lookup, dir_lookup) abort
 
     " Search for an exact file match first.
     let l:file_list = get(a:file_lookup, l:buffer_filename, [])
+
+    " We may have to look for /foo/bar instead of C:\foo\bar
+    if empty(l:file_list) && has('win32')
+        let l:file_list = get(
+        \   a:file_lookup,
+        \   ale#path#RemoveDriveLetter(l:buffer_filename),
+        \   []
+        \)
+    endif
+
     " Try the absolute path to the directory second.
     let l:dir_list = get(a:dir_lookup, l:dir, [])
 
+    if empty(l:dir_list) && has('win32')
+        let l:dir_list = get(
+        \   a:dir_lookup,
+        \   ale#path#RemoveDriveLetter(l:dir),
+        \   []
+        \)
+    endif
+
     if empty(l:file_list) && empty(l:dir_list)
         " If we can't find matches with the path to the file, try a
         " case-insensitive match for any similarly-named file.
@@ -378,6 +396,14 @@ function! ale#c#ParseCompileCommandsFlags(buffer, file_lookup, dir_lookup) abort
             let l:key = fnamemodify(l:buffer_filename, ':r') . l:suffix
             let l:file_list = get(a:file_lookup, l:key, [])
 
+            if empty(l:file_list) && has('win32')
+                let l:file_list = get(
+                \   a:file_lookup,
+                \   ale#path#RemoveDriveLetter(l:key),
+                \   []
+                \)
+            endif
+
             if empty(l:file_list)
                 " Look fuzzy matches on the basename second.
                 let l:key = fnamemodify(l:basename, ':r') . l:suffix
@@ -412,7 +438,8 @@ function! ale#c#ParseCompileCommandsFlags(buffer, file_lookup, dir_lookup) abort
     for l:item in l:dir_list
         let l:filename = ale#path#GetAbsPath(l:item.directory, l:item.file)
 
-        if ale#path#Simplify(fnamemodify(l:filename, ':h')) is? l:dir
+        if ale#path#RemoveDriveLetter(fnamemodify(l:filename, ':h'))
+        \  is? ale#path#RemoveDriveLetter(l:dir)
             let [l:should_quote, l:args] = s:GetArguments(l:item)
 
             return ale#c#ParseCFlags(l:item.directory, l:should_quote, l:args)

autoload/ale/path.vim

@@ -24,6 +24,14 @@ function! ale#path#Simplify(path) abort
     return substitute(simplify(l:win_path), '^\\\+', '\', 'g') " no-custom-checks
 endfunction
 
+" Simplify a path without a Windows drive letter.
+" This function can be used for checking if paths are equal.
+function! ale#path#RemoveDriveLetter(path) abort
+    return has('win32') && a:path[1:2] is# ':\'
+    \   ? ale#path#Simplify(a:path[2:])
+    \   : ale#path#Simplify(a:path)
+endfunction
+
 " Given a buffer and a filename, find the nearest file by searching upwards
 " through the paths relative to the given buffer.
 function! ale#path#FindNearestFile(buffer, filename) abort

test/test_c_flag_parsing.vader

@@ -419,7 +419,7 @@ Execute(ShellSplit should not merge flags):
   \   '-Idir-with-dash',
   \   'subdir/somedep3.o',
   \   'subdir/somedep4.o',
-  \   '-Ikernel/include',
+  \   '-I' . ale#path#Simplify('kernel/include'),
   \   'subdir/somedep5.o',
   \   'subdir/somedep6.o',
   \ ],