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.

Reviewers: javed.absar

Subscribers: dexonsmith, llvm-commits

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341728 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed