[clang] [clang] Add new warning: not eliding copy on return (missed NRVO) (PR #139973)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Thanks. This needs a few more tests.

* A few tests to make sure we don't produce false positives.
* Tests with function templates with placeholder return types.
  We should never be performing copy elision for these, due to implementation limitations.

https://github.com/llvm/llvm-project/pull/139973