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

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

The issue above seems to be a clang bug to me according to https://cplusplus.github.io/CWG/issues/1467.html

List-initialization of an object or reference of type T is defined as follows:

If T is a class type and the initializer list has a single element of type cv T or a class type derived from T, the object is initialized from that element.

Therefore `b` should be initialized from `a` instead of `A(a)`.

https://github.com/llvm/llvm-project/pull/77768