Promote storage-only __fp16 vector operands to float vectors.

This commit fixes a bug in the handling of storage-only __fp16 vectors
where clang didn't promote __fp16 vector operands to float vectors.

Conceptually, it performs the following transformation on the AST in
CreateBuiltinBinOp and CreateBuiltinUnaryOp:

(Before)
  typedef __fp16 half4 __attribute__ ((vector_size (8)));
  typedef float float4 __attribute__ ((vector_size (16)));
  half4 hv0, hv1, hv2, hv3;

  hv0 = hv1 + hv2 + hv3;

(After)
  float4 t0 = (float4)hv1 + (float4)hv2;
  float4 t1 = t0 + (float4)hv3;
  hv0 = (half4)t1;

Note that this commit fixes the bug for targets that set
HalfArgsAndReturns to true (ARM and ARM64). Targets using intrinsics
such as llvm.convert.to.fp16 to handle __fp16 are still broken.

rdar://problem/20625184

Differential Revision: https://reviews.llvm.org/D32520

llvm-svn: 314056