[ARM] Moving CMSE handling of half arguments and return to the backend

Summary:
As half-precision floating point arguments and returns were previously
coerced to either float or int32 by clang's codegen, the CMSE handling
of those was also performed in clang's side by zeroing the unused MSBs
of the coercer values.

This patch moves this handling to the backend's calling convention
lowering, making sure the high bits of the registers used by
half-precision arguments and returns are zeroed.

Reviewers: chill, rjmccall, ostannard

Reviewed By: ostannard

Subscribers: kristof.beyls, hiraditya, danielkiss, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D81428