[clang] [Clang] Transform lambda's constraints when instantiating parameter mapping (PR #195995)

[Matheus Izvekov via cfe-commits]

================
@@ -1876,11 +1901,25 @@ namespace {
                               TemplateParameterList *OrigTPL)  {
       if (!OrigTPL || !OrigTPL->size()) return OrigTPL;
 
+      std::optional<MultiLevelTemplateArgumentList> OldMLTAL;
+      // We need to preserve the lambda depth in parameter mapping.
+      // Otherwise the template argument deduction would fail, if we reduced the
+      // depth too early.
+      if (SemaRef.inParameterMappingSubstitution() &&
+          getDepthAndIndex(OrigTPL->getParam(0)).first >=
+              TemplateArgs.getNumSubstitutedLevels())
+        OldMLTAL = ForgetSubstitution();
----------------
mizvekov wrote:

Would look cleaner if this used `ForgetSubstitutionRAII`.

Also, I don't see how the level check is necessary. The comment doesn't explain it either.

https://github.com/llvm/llvm-project/pull/195995