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

[via cfe-commits]

Sirraide wrote:

> I realized we have a `ExprConstShared.h` so I put a function with the `!FD->isDefined() && FD->isImplicitlyInstantiable() && FD->isConstexpr()` in there

Oh I didn’t realise that that was a thing either; stgm.

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