[MS Demangler] Demangle string literals.

When demangling string literals, Microsoft's undname
simply prints 'string'.  This patch implements string
literal demangling while doing a bit better than this
by decoding as much of the string as possible and
trying to faithfully reproduce the original string
literal definition.

This is a bit tricky because the different character
types char, char16_t, and char32_t are not uniquely
identified by the mangling, so we have to use a
heuristic to try to guess the character type.  But
it works pretty well, and many tests are added to
illustrate the behavior.

Differential Revision: https://reviews.llvm.org/D50806

llvm-svn: 339892