Add clang to cpp linters.

ale_linters/cpp/clang.vim

@@ -0,0 +1,20 @@
+" Author: Tomota Nakamura <https://github.com/tomotanakamura>
+" Description: clang linter for cpp files
+
+" Set this option to change the Clang options for warnings for CPP.
+if !exists('g:ale_cpp_clang_options')
+    " let g:ale_cpp_clang_options = '-Wall'
+    " let g:ale_cpp_clang_options = '-std=c99 -Wall'
+    " c11 compatible
+    let g:ale_cpp_clang_options = '-std=c11 -Wall'
+endif
+
+call ale#linter#Define('cpp', {
+\   'name': 'clang',
+\   'output_stream': 'stderr',
+\   'executable': 'clang++',
+\   'command': 'clang++ -S -x c -fsyntax-only '
+\       . g:ale_cpp_clang_options
+\       . ' -',
+\   'callback': 'ale#handlers#HandleGCCFormat',
+\})