[ARM][Clang] Removing lowering of half-precision FP arguments and returns from Clang's CodeGen

Summary:
On the process of moving the argument lowering handling for
half-precision floating point arguments and returns to the backend, this
patch removes the code that was responsible for handling the coercion of
those arguments in Clang's Codegen.

Reviewers: rjmccall, chill, ostannard, dnsampaio

Reviewed By: ostannard

Subscribers: stuij, kristof.beyls, dmgreen, danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81451