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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This seems reaosnable to me? Or at least as we can be for what sort of stuff this needs to do.

Though I DO wonder if the property:
`!FD->isDefined() && FD->isImplicitlyInstantiable() && FD->isConstexpr()`

should be a function inside of `FunctionDecl` with its own descriptive name.  Those conditions there are pretty horrific to read.

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