CodeGen: Cleanup MS ABI RTTI LLVM IR

The MS ABI RTTI emission code would choose names for IR types like
%"MSRTTITypeDescriptor\02".  This name is undesirable because it
requires escaping; the underlying reason for this is that the name is
unprintable.  Fix this by naming it %rtti.TypeDescriptor2.

While here, stop trying to do lookups in the LLVM Module's type table.
Instead, store the IR types in MicrosoftCXXABI.  Lookups by name aren't
particularly fast.