[-fms-extensions] Add support for __FUNCDNAME__

Summary:
Similar to __FUNCTION__, MSVC exposes the name of the enclosing mangled
function name via __FUNCDNAME__.  This implementation is very naive and
unoptimized, it is expected that __FUNCDNAME__ would be used rarely in
practice.

Reviewers: rnk, rsmith, thakis

CC: cfe-commits, silvas

Differential Revision: http://llvm-reviews.chandlerc.com/D2109