[clang][BFloat] add NEON emitter for bfloat

Summary:
This patch adds the bfloat16_t struct typedefs (e.g. bfloat16x8x2_t) to
arm_neon.h

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 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
- Simon Tatham
- Ties Stuij

Reviewers: t.p.northover, fpetrogalli, sdesmalen, az, LukeGeeson

Reviewed By: fpetrogalli

Subscribers: SjoerdMeijer, LukeGeeson, pbarrio, mgorny, kristof.beyls, ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79708