Teach ARM va_arg to ignore empty structs.

Empty structs are ignored for parameter passing purposes, but va_arg was
incrementing the pointer anyway which could lead to va_list getting out of
sync.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184605 91177308-0d34-0410-b5e6-96231b3b80d8