[clang][BFloat] Add create/set/get/dup intrinsics

Summary:
This patch is part of a series that adds support for 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 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:
- Luke Cheeseman
- Momchil Velikov
- Luke Geeson
- Ties Stuij
- Mikhail Maltsev

Reviewers: t.p.northover, sdesmalen, fpetrogalli, LukeGeeson, stuij, labrinea

Reviewed By: labrinea

Subscribers: miyuki, dmgreen, labrinea, kristof.beyls, ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79710