[ms] Fix mangling of char16_t and char32_t to be compatible with MSVC.

MSVC limits char16_t and char32_t string literal names to 32 bytes of character
data, not to 32 characters. wchar_t string literal names on the other hand can
get up to 64 bytes of character data.

https://reviews.llvm.org/D48781

llvm-svn: 336097