[clang] [Clang] Instantiate constexpr function when they are needed. (PR #173537)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 commented:

There are https://github.com/llvm/llvm-project/issues/35052 and related issues. Are they fixed now?

https://github.com/llvm/llvm-project/pull/173537