Proper va_arg/va_copy lowering on win64

Win64 uses CharPtrBuiltinVaList instead of X86_64ABIBuiltinVaList like
other 64-bit targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187355 91177308-0d34-0410-b5e6-96231b3b80d8