[clang] [Clang] Instantiate local constexpr functions eagerly (PR #95660)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

It's still broken, and I haven’t figured out an approach to fix the regression.

I do recall that we were planning to wire up Sema and the constant evaluator, so this patch might end up being completely superseded if that gets implemented.

https://github.com/llvm/llvm-project/pull/95660