[Sema] Use the canonical type in function isVector

This reapplies the following patch, which was reverted because it caused
neon CodeGen tests to fail:

https://reviews.llvm.org/rGa6150b48cea00ab31e9335cc73770327acc4cb3a

I've added checks to detect half precision neon vectors and avoid
promiting them to vectors of floats.

See the discussion here: https://reviews.llvm.org/rG825235c140e7

Original commit message:

This fixes an assertion in Sema::CreateBuiltinBinOp that fails when one
of the vector operand's element type is a typedef of __fp16.

rdar://problem/55983556