[clang] [clang] Track final substitution for Subst* AST nodes (PR #132748)

[Thurston Dang via cfe-commits]

thurstond wrote:

> @fmayer this patch touches nowhere near where those backtraces indicate. #130182 is a far more likely culprit.

 #130182 cannot be the cause of the ongoing MSan buildbot failures, because it was already reverted two days ago in #134239

https://github.com/llvm/llvm-project/pull/132748