`std::bit_cast` participates in overload resolution only if `sizeof(To) == sizeof(From)` and both `To` and `From` are *TriviallyCopyable* types, so the static assertions here can be removed. `[[nodiscard]]` was added as well.

With the upgrade to C++20, std::countl_zero and std::countr_zero can replace these home-spun implementations from the BitUtil.h library.

Now that we've flipped the C++20 switch, let's start making use of
the nice new <bit> header.

I'm planning on handling this move away from BitUtils.h incrementally
in a series of PRs. There may be a few functions remaining in
BitUtils.h by the end that C++20 doesn't have any equivalents for.

C++23 deprecates std::aligned_storage_t while alignas works since C++11.

This fixes issue 12925.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.