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

[John McCall via cfe-commits]

rjmccall wrote:

I get the idea here, but I feel like being able to arbitrarily re-enter Sema during constant evaluation is going to be a huge source of bugs. An alternative design would be add another level of used-ness that forces instantiation of `constexpr` used functions, right?

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