[analyzer] False negatives of uninitialized variables in C++ for record types after self-assignment initialization

Continuing #187530 for the false negatives detected in the test cases.

1. Unions in C++
https://github.com/llvm/llvm-project/blob/4aa1590ad66a1bff9b3c74b4a2c7366473015b4c/clang/test/Analysis/issue-173210-self-assign-init.c#L48-L55

2. Self-assignment in default member initializer
https://github.com/llvm/llvm-project/blob/4aa1590ad66a1bff9b3c74b4a2c7366473015b4c/clang/test/Analysis/issue-173210-self-assign-init.c#L68-L74

We can discuss here whether the analyzer should report them.

Start with clang/test/Analysis/issue-173210-self-assign-init.c, especially lines 48-55 and 68-74, and compare the existing analyzer behavior with issue #187530. Determine whether unions and self-assignment in default member initializers should produce diagnostics, then confirm the expected behavior with maintainers. Done means the decision is recorded and the relevant regression coverage and analyzer behavior agree.

Written by the indexing model from the issue text.