I'm still "chewing on this", I'm not sure I fully understand the problem well enough to give great "big picture" feedback. In any case, I've left a few comments where I had a potentially helpful thought :)

Formatting fixes

Generally agree with this direction; Are there plans for migrating https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Basic/Specifiers.h in a similar fashion, for consistency?

+1 Nice QOL change :)

Abandoned in favor of https://reviews.llvm.org/D76420

Thanks Tyker, you're right, there is a general problem that needs handled, if this is to be handled; I had a bit of tunnel vision here. However, I'm retracting/abandoning this patch entirely as upon further review, it's not standard compliant. The current situation, is actually the correct one (for now?).

Updated to remove duplicated (and out dated) APValue generation logic, and fixed formatting issues. (Thanks for pointing out something was awry Tyker)

Updated the patch to correct formatting issues, and removed application of the ConstantEvaluated evaluation context to var decls as it introduces too much complexity.

Dropped the override for constexpr evaluator, LValue evaluation base class

I should note, I don't have commit access, so I'm unable to commit this myself. Attribution would be Wyatt Childers <wchilders@lock3software.com>

Updated to assume LValue ConstantExprs have LValue APValues, with verification in debug mode. Additionally, added a missing LValue -> RValue conversion for VerifyIntegerConstantExpression.

In D76396#1930773, @Tyker wrote:

I have already a patch aiming to do the same thing. D74007