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

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

LGTM. Thanks!

Please wait until next week to merge.
In particular, I want to make sure @shafik and @zygoloid are happy with the PR.

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