[MS ABI] Cleanup our mangling of vector types

We used to produce a type which demangled to:
union __clang_vec8_F

That 'F' is the mangling for 'short' but it is present in the mangled
name in an inappropriate place, leading to it not getting demangled.

Instead, create a synthetic class type in a synthetic namespace called
__clang. With this, it now demangles to:
union __clang::__vector<short,8>

llvm-svn: 256556