[Clang] [P2025] More exhaustive tests for NRVO

This is a preliminary patch ahead of D119792 (I'll rebase that one on top of this).
This shows what Clang's _current_ behaviour is for calculating NRVO in various
common cases. Then, in D119792 (and future patches), I'll be able to demostrate
exactly how LLVM IR for each of these cases changes.

Reviewed By: Quuxplusone

Differential Revision: https://reviews.llvm.org/D119927