Also promote fp16 types to double when they're anonymous variadic arguments.

__fp16 isn't covered by the standard, but this resolves the oddity that float
gets promoted when passed variadically, but not the smaller type. This is
required by the AArch64 ABI, and a sane action elsewhere.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173918 91177308-0d34-0410-b5e6-96231b3b80d8