[MS Demangler] Fix a few more edge cases.

I found these by running llvm-undname over a couple hundred
megabytes of object files generated as part of building chromium.
The issues fixed in this patch are:

  1) decltype-auto return types.
  2) Indirect vtables (e.g. const A::`vftable'{for `B'})
  3) Pointers, references, and rvalue-references to member pointers.

I have exactly one remaining symbol out of a few hundred MB of object
files that produces a name we can't demangle, and it's related to
back-referencing.

llvm-svn: 340341