MS ABI: Mangle char16_t and char32_t string literals

We previously had support for char and wchar_t string literals.  VS 2015
added support for char16_t and char32_t.

String literals must be mangled in the MS ABI in order for them to be
deduplicated across translation units: their linker has no notion of
mergeable section.  Instead, they use the mangled name to make a COMDAT
for the string literal; the COMDAT will merge with other COMDATs in
other object files.

This allows strings in object files generated by clang to get merged
with strings in object files generated by MSVC.