[Clang] [P2025] Analyze only potential scopes for NRVO

Before the patch we calculated the NRVO candidate looking at the
variable's whole enclosing scope. The research in [P2025] shows that
looking at the variable's potential scope is better and covers more
cases where NRVO would be safe and desirable.

Many thanks to @Izaron for the original implementation.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119792