Add support for MSVC's __FUNCSIG__

It is very similar to GCC's __PRETTY_FUNCTION__, except it prints the
calling convention.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D3311

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205780 91177308-0d34-0410-b5e6-96231b3b80d8