ARM: teach Sema that "r" can match 64-bit values

We already support using "r" on 64-bit values (a GPRPair is
allocated), but Sema doesn't know this yet so issues a warning. This
should fix it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196724 91177308-0d34-0410-b5e6-96231b3b80d8