[IR][BFloat] Add BFloat IR type

Summary:
The BFloat IR type is introduced to provide support for, initially, the BFloat16
datatype introduced with the Armv8.6 architecture (optional from Armv8.2
onwards). It has an 8-bit exponent and a 7-bit mantissa and behaves like an IEEE
754 floating point IR type.

This is part of a patch series upstreaming Armv8.6 features. Subsequent patches
will upstream intrinsics support and C-lang support for BFloat.

Reviewers: SjoerdMeijer, rjmccall, rsmith, liutianle, RKSimon, craig.topper, jfb, LukeGeeson, sdesmalen, deadalnix, ctetreau

Subscribers: hiraditya, llvm-commits, danielkiss, arphaman, kristof.beyls, dexonsmith

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78190