[-cxx-abi microsoft] Mangle local TagDecls appropriately

Summary:
When selecting a mangling for an anonymous tag type:
- We should first try it's typedef'd name.
- If that doesn't work, we should mangle in the name of the declarator
  that specified it as a declaration specifier.
- If that doesn't work, fall back to a static mangling of
  <unnamed-type>.

This should make our anonymous type mangling compatible.

This partially fixes PR16994; we would need to have an implementation of
scope numbering to get it right (a separate issue).

Reviewers: rnk, rsmith, rjmccall, cdavis5x

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1540