[BFloat] Add convert/copy instrinsic support

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

Specifically it adds intrinsic support in clang and llvm for Arm and AArch64.

The bfloat type, and its properties are specified in the Arm Architecture Reference Manual:

https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile

The following people contributed to this patch:
  - Alexandros Lamprineas
  - Luke Cheeseman
  - Mikhail Maltsev
  - Momchil Velikov
  - Luke Geeson

Differential Revision: https://reviews.llvm.org/D80928