[ARM][bfloat] Do not coerce bfloat arguments and returns to integers

Summary:
As part of moving the argument lowering handling for bfloat 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.

Subscribers: kristof.beyls, danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81837