Implement SparcV9ABIInfo::EmitVAArg.

This could actually be implemented with the LLVM IR va_arg instruction,
but it doesn't seem to offer any advantages over accessing the va_list
pointer directly.

Using the va_list pointer directly makes it possible to perform type
coercion directly from the argument array, and the va_list updates are
exposed to the optimizers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183292 91177308-0d34-0410-b5e6-96231b3b80d8