Fix #1608 - Treat .h as C++ files for C++ clang-tidy

ale_linters/cpp/clangtidy.vim

@@ -25,6 +25,11 @@ function! ale_linters#cpp#clangtidy#GetCommand(buffer, output) abort
         let l:options .= !empty(l:options) ? ale#Pad(l:cflags) : l:cflags
     endif
 
+    " Tell clang-tidy a .h header with a C++ filetype in Vim is a C++ file.
+    if expand('#' . a:buffer) =~# '\.h$'
+        let l:options .= !empty(l:options) ? ' -x c++' : '-x c++'
+    endif
+
     " Get the options to pass directly to clang-tidy
     let l:extra_options = ale#Var(a:buffer, 'cpp_clangtidy_extra_options')
 

test/command_callback/test_clang_tidy_command_callback.vader

@@ -68,6 +68,7 @@ Execute(The build directory should be used for header files):
   \ ale#Escape('clang-tidy')
   \   . ' -checks=' . ale#Escape('*') . ' %s'
   \   . ' -p ' . ale#Escape('/foo/bar')
+  \   . ' -- -x c++'
 
   call ale#test#SetFilename('test.hpp')