[clang] [Clang][AST] {CXXDefaultArgExpr, CXXDefaultInitExpr}::hasRewrittenInit return true iff the init expression was really rebuild (PR #99748)

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yronglin wrote:

Thanks for the review! Hmm, this patch was splitted from https://github.com/llvm/llvm-project/pull/91879. In that patch, we should guarantee the sub-expression was actually rebuilt when CXXDefaultInitExpr::hasRewrittenInit() returns true. But https://github.com/llvm/llvm-project/pull/91879 and the dependency patch https://github.com/llvm/llvm-project/pull/87933 was reverted, so I'd like to implement this in a seprate patch. [Reapply CWG1815 patch](https://github.com/llvm/llvm-project/pull/97308) was used to reapply CWG1815, and I'll also reapply  https://github.com/llvm/llvm-project/pull/91879 once CWG1815 landed. I'd like to preserve the history of my commit in [Reapply CWG1815 patch](https://github.com/llvm/llvm-project/pull/97308) patch to make it's easily to review. [Reapply CWG1815 patch](https://github.com/llvm/llvm-project/pull/97308) still needs more improve, but I will finish it as soon as possible.

https://github.com/llvm/llvm-project/pull/99748