[clang][BFloat] Add reinterpret cast intrinsics

Summary:
This patch is part of a series implementing the Bfloat16 extension of the
Armv8.6-a architecture, as detailed here:

https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/arm-architecture-developments-armv8-6-a

The bfloat type, and its properties is specified in the Arm C language
extension specification:

https://developer.arm.com/docs/ihi0055/d/procedure-call-standard-for-the-arm-64-bit-architecture

Subscribers: kristof.beyls, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79869

The following people contributed to this patch:

- Luke Cheeseman
- Alexandros Lamprineas
- Luke Geeson
- Ties Stuij