[NFC] [C++20] [Modules] Code cleanups when checking modules in ADL

The original code uses getTopLevelModuleName to compare if the module
matches. This is an overkill. Since in one program there will only be at
most one module name. So it is good enough to compare the module
directly. So that we can save some string comparisons.

clang/lib/Sema/SemaLookup.cpp

@@ -3894,9 +3894,7 @@ void Sema::ArgumentDependentLookup(DeclarationName Name, SourceLocation Loc,
                   // ... and have the same innermost enclosing non-inline
                   // namespace scope as a declaration of an associated entity
                   // attached to M
-                  if (!E->hasOwningModule() ||
-                      E->getOwningModule()->getTopLevelModuleName() !=
-                          FM->getTopLevelModuleName())
+                  if (E->getOwningModule() != FM)
                     return false;
                   // TODO: maybe this could be cached when generating the
                   // associated namespaces / entities.