Mangle calling conventions into function pointer types where GCC does

Summary:
GCC 5.1 began mangling these Windows calling conventions into function
types, since they can be used for overloading. They've always been
mangled in the MS ABI, but they are new to the Itanium mangler. Note
that the calling convention doesn't appear as part of the main
declaration, it only appears on function parameter types and other
types.

Fixes PR39860

Reviewers: rjmccall, efriedma

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55672

llvm-svn: 349212