Add a stub mangling for ObjC selectors in the Microsoft ABI.

This mangling is used only for outlined SEH finally blocks, which have
internal linkage.

This fixes the failure of CodeGenObjC/2007-04-03-ObjcEH.m on builds with
expensive checks enabled, on Windows.  This test should probably be
specifying a triple: it currently picks up whatever the host environment
is using.  Unfortunately, I have no idea what it is trying to test,
because it contains no comments and predates Clang having working
Objective-C IR generation.

llvm-svn: 339667