Add frontend support for __vectorcall

Wire it through everywhere we have support for fastcall, essentially.

This allows us to parse the MSVC "14" CTP headers, but we will
miscompile them because LLVM doesn't support __vectorcall yet.

Reviewed By: Aaron Ballman

Differential Revision: http://reviews.llvm.org/D5808

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220573 91177308-0d34-0410-b5e6-96231b3b80d8