ADT: add <bit> header, implement C++20 bit_cast, use

Summary: I saw a few places that were punning through a union of FP and integer, and that made me sad. Luckily, C++20 adds bit_cast for exactly that purpose. Implement our own version in ADT (without constexpr, leaving us a bit sad), and use it in the few places my grep-fu found silly union punning.

This was originally committed as r341728 and reverted in r341730.

Reviewers: javed.absar, steven_wu, srhines

Subscribers: dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D51693

llvm-svn: 341741