[clang] Reapply "[Analyzer][CFG] Correctly handle rebuilt default arg and default init expression" (PR #127338)

[via cfe-commits]

yronglin wrote:

I don't know whey we stripped off the top level `FullExpr` and `ConstantExpr` in `CXXDefaultArgExpr::getExpr`, but not in `CXXDefaultInitExpr::getExpr`. Do you know about this? @cor3ntin 

https://github.com/llvm/llvm-project/pull/127338