I've pushed a merge with main to resolve a simple conflict with #4000, then a whole bunch of changes for issues identified during code review (a few were pre-existing cleanups that fit with this PR):

• Add missing defined.
• Restore lost underscores.
• Add missing arrow, replace complex De Morgan with matching comment.
• Fix whitespace.
• Use proper arrow comments.
• Simply drop a horribly long comment, using our convention that we can omit them in single-line, non-nested contexts.
• Too many arrows.
• Use systematic _HAS_CXX17 comments.
• Use systematic _HAS_CXX20 comments.
• Use matching arrow comments.
• Simplify calling convention comments, omit pre-existing newlines.
• Fix comments around extended aligned storage.
• Drop spurious !.
• Add missing !.
• Fix damaged !!, use matching "floating-point exceptions disabled (default)" comment.
• Fix FBITS comments.

These comment changes aren't risky; we can merge them without a second maintainer approval.

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

Thanks again for improving codebase consistency! 📜 📄 ✨