Revert "[SemaCXX] Implement CWG2137 (list-initialization from objects of the same type) (#77768)" in release/18.x (#79400)

- Revert "[SemaCXX] Implement CWG2137 (list-initialization from objects
of the same type) (#77768)", see
#77768 (comment)

Author: alexfh

clang/docs/ReleaseNotes.rst

@@ -234,11 +234,6 @@ C++2c Feature Support
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- Implemented `CWG2137 <https://wg21.link/CWG2137>`_ which allows
-  list-initialization from objects of the same type.
-- Implemented `CWG2311 <https://wg21.link/CWG2311>`_: given a prvalue ``e`` of object type
-  ``T``, ``T{e}`` will try to resolve an initializer list constructor and will use it if successful (CWG2137).
-  Otherwise, if there is no initializer list constructor, the copy will be elided as if it was ``T(e)``.
 
 - Implemented `CWG2598 <https://wg21.link/CWG2598>`_ and `CWG2096 <https://wg21.link/CWG2096>`_,
   making unions (that have either no members or at least one literal member) literal types.