[git-clang-format] recognize hxx as a C++ file

clangd, clang-tidy, etc does that already, no reason why
git-clang-format should skip hxx files.

Reviewed By: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D63621

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@364014 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
1 file changed