[MS Demangler] Resolve back-references lazily.

We need to both record and resolve back-references lazily due to
not being able to know until a demangling is complete whether or
not a name should go into the back-reference table..  This patch
implements lazy resolution of back-references, but we still have
eager recording of back-references.  This will be fixed in a
subsequent patch.

llvm-svn: 338736