MS ABI: Mangle char16_t and char32_t types

These mangling make clang more compatible with MSVC 2015.
Correctly mangling char16_t and char32_t will take a little more work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222515 91177308-0d34-0410-b5e6-96231b3b80d8