[clang] [clang] Fix assertion failure in constexpr union deserialization (PR #140179)

[via cfe-commits]

================
@@ -414,7 +414,7 @@ let Class = PropertyTypeCase<APValue, "Union"> in {
     let Read = [{ node.getUnionValue() }];
   }
   def : Creator<[{
-    return APValue(cast<clang::FieldDecl>(fieldDecl), std::move(value));
+    return APValue(cast_if_present<clang::FieldDecl>(fieldDecl), std::move(value));
----------------
cor3ntin wrote:

It might be worth adding a comment in the code

https://github.com/llvm/llvm-project/pull/140179