[SemaCXX] Implement CWG2137 (list-initialization from objects of the same type) (#77768)

Closes #77638, #24186

Rebased from <https://reviews.llvm.org/D156032>, see there for more
information.

Implements wording change in [CWG2137](https://wg21.link/CWG2137) in the
first commit.

This also implements an approach to [CWG2311](https://wg21.link/CWG2311)
in the second commit, because too much code that relies on `T{ T_prvalue}` 
being an elision would break. Because that issue is still open and
the CWG issue doesn't provide wording to fix the issue, there may be
different behaviours on other compilers.