Fix FindCompileCommands to handle absolute paths in c_build_dir_names

Use ale#path#GetAbsPath to resolve build directory paths, which correctly
handles both absolute and relative paths. When the dirname is absolute,
derive the project root from the parent of the build directory.

Co-authored-by: w0rp <3518142+w0rp@users.noreply.github.com>

test/test_c_find_compile_commands.vader

@@ -0,0 +1,33 @@
+Before:
+  Save g:ale_c_build_dir_names
+
+  call ale#test#SetDirectory('/testplugin/test')
+
+After:
+  Restore
+
+  call ale#test#RestoreDirectory()
+
+Execute(FindCompileCommands should find compile_commands.json with relative build dir names):
+  call ale#test#SetFilename('test-files/c/json_project/subdir/dummy')
+
+  let g:ale_c_build_dir_names = ['build']
+
+  AssertEqual
+  \ [
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project'),
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project/build/compile_commands.json'),
+  \ ],
+  \ ale#c#FindCompileCommands(bufnr(''))
+
+Execute(FindCompileCommands should find compile_commands.json with absolute build dir names):
+  call ale#test#SetFilename('test-files/c/json_project/subdir/dummy')
+
+  let g:ale_c_build_dir_names = [ale#path#Simplify(g:dir . '/test-files/c/json_project/build')]
+
+  AssertEqual
+  \ [
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project'),
+  \   ale#path#Simplify(g:dir . '/test-files/c/json_project/build/compile_commands.json'),
+  \ ],
+  \ ale#c#FindCompileCommands(bufnr(''))