[clang] [Clang] Instantiate functions from constant evaluation. (PR #205557)

[Daniel M. Katz via cfe-commits]

katzdm wrote:

There are two outstanding cases (detailed above) re: the "what if we remove early instantiation of constexpr functions" experiment from @jyknight that I'm still hoping to find time to investigate. Other than that, I think this is ready for another review pass.

https://github.com/llvm/llvm-project/pull/205557