MS ABI: Add CodeGen support for rethrowing MS C++ exceptions

Rethrowing exceptions in the MS model is very simple: just call
_CxxThrowException with nullptr for both arguments.

N.B.  They chose stdcall as the calling convention for x86 but cdecl for
all other platforms.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@222733 91177308-0d34-0410-b5e6-96231b3b80d8