[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: https://llvm.org/svn/llvm-project/cfe/trunk@364014 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed