[clang] [Clang][Sema] fix crash of attribute transform (PR #78088)

[Aaron Ballman via cfe-commits]

================
@@ -6124,7 +6124,11 @@ QualType TreeTransform<Derived>::TransformFunctionProtoType(
       //   "pointer to cv-qualifier-seq X" between the optional cv-qualifer-seq
       //   and the end of the function-definition, member-declarator, or
       //   declarator.
-      Sema::CXXThisScopeRAII ThisScope(SemaRef, ThisContext, ThisTypeQuals);
+      auto *RD =
+          dyn_cast_or_null<CXXRecordDecl>(SemaRef.getCurLexicalContext());
+      Sema::CXXThisScopeRAII ThisScope(
+          SemaRef, ThisContext == nullptr && nullptr != RD ? RD : ThisContext,
+          ThisTypeQuals);
----------------
AaronBallman wrote:

```suggestion
      auto *RD =
          dyn_cast<CXXRecordDecl>(SemaRef.getCurLexicalContext());
      Sema::CXXThisScopeRAII ThisScope(
          SemaRef, !ThisContext && RD ? RD : ThisContext,
          ThisTypeQuals);
```

https://github.com/llvm/llvm-project/pull/78088