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

[Thurston Dang via cfe-commits]

thurstond wrote:

> @thurstond Well can you provide a more recent buildbot failure link where #130182 is not on the list of possible culpirits? Because the link @fmayer provided for sure included it.

@mizvekov I localized the MSan error to this patch by re-running the buildbot script with this patch and immediately prior, in both cases also reverting #130182 to remove noise:

Repro:

```
cd ~/llvm-project
# This patch
git checkout f302f35526553abcb46dab278c4494c3d01deb45
# Revert #130182
git revert 76fa9530c9ac7f81a49b840556f51f4838efbfe1

# Buildbot
mkdir ~/buildbot_repro
cd ~/buildbot_repro
git clone https://github.com/llvm/llvm-zorg.git
BUILDBOT_MONO_REPO_PATH=~/llvm-project bash llvm-zorg/zorg/buildbot/builders/sanitizers/buildbot_fast.sh
# For speed, I commented out build_stage2_asan_ubsan and check_stage2_asan_ubsan in the script above
```

This repro'ed the MSan failures:
```
  Clang :: ASTMerge/class-template-partial-spec/test.cpp
  Clang :: Analysis/ctu-inherited-default-ctor.cpp
```

The MSan failures are not present when using the immediately prior patch:
```
# Immediately prior to this patch
git checkout 990a086d9da0bc2fd53a6a4c95ecbbe23a297a83
git revert 76fa9530c9ac7f81a49b840556f51f4838efbfe1

# Same buildbot steps as before
```

Please let me know if you encounter any issues reproducing the error.


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