[ms] Fix mangling of vector types in QMM_Result contexts.

If QMM_Result is set (which it is for return types, RTTI descriptors, and
exception type descriptors), tag types (structs, enums, classes, unions) get
their qualifiers mangled in.

__m64 and friends is a struct/union thingy in MSVC, but not in clang's headers.
To make mangling work, we call mangleArtificalTagType(TTK_Union/TTK_Struct for
the vector types to mangle them as tag types -- but the isa<TagType> check when
mangling in QMM_Result mode isn't true for these vector types. Add an
isArtificialTagType() function and check for that too. Fixes PR37276 and some
other issues.

I tried to audit all references to TagDecl and TagType in MicrosoftMangle.cpp
to find other places where we need to call mangleArtificalTagType(), but
couldn't find any.

I tried to audit all calls to mangleArtificalTagType() to see if
isArtificialTagType() needs to handle more than just the vector types, but as
far as I can tell all other types we use it for are types that MSVC can't
handle at all (Objective-C types etc).

https://reviews.llvm.org/D49597

llvm-svn: 337732