[CodeGen] Properly support the half FP type with non-native operations.

On AArch64, the -fallow-half-args-and-returns option is the default.
With it, the half type is considered legal (rather than the i16 used
normally for __fp16), but no operation is, except conversions and
load/stores and such.

The previous behavior was tantamount to saying LangOpts.NativeHalfType
was implied by LangOpts.HalfArgsAndReturns, which isn't true.
Instead, teach the various parts of CodeGen that already know about
half (using the intrinsics or not) about this weird in-between case,
where the "half" type is legal, but operations on it aren't.

This is a smaller intermediate step to the end-goal of removing the
intrinsic, always using "half", and letting the backend legalize.

Builds on r232968.
rdar://20045970, rdar://17468714
Differential Revision: http://reviews.llvm.org/D8367

llvm-svn: 232971