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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

@fmayer this patch touches nowhere near where those backtraces indicate.
https://github.com/llvm/llvm-project/pull/130182 is a far more likely culprit.

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