Sema: Avoid adding nonisolated twice to synthesized Hashable methods.

https://github.com/apple/swift/pull/42041 introduced a centralized mechanism for adding the `nonisolated` attribute to synthesized decls but did not clean up the existing code that was already doing so for `Hashable` conformances.

Resolves rdar://102106591

lib/Sema/CodeSynthesis.cpp

@@ -1605,13 +1605,14 @@ bool swift::hasLetStoredPropertyWithInitialValue(NominalTypeDecl *nominal) {
   });
 }
 
-void swift::addNonIsolatedToSynthesized(
-    NominalTypeDecl *nominal, ValueDecl *value) {
+bool swift::addNonIsolatedToSynthesized(NominalTypeDecl *nominal,
+                                        ValueDecl *value) {
   if (!getActorIsolation(nominal).isActorIsolated())
-    return;
+    return false;
 
   ASTContext &ctx = nominal->getASTContext();
   value->getAttrs().add(new (ctx) NonisolatedAttr(/*isImplicit=*/true));
+  return true;
 }
 
 static std::pair<BraceStmt *, /*isTypeChecked=*/bool>