Fix synthesis of 'override' and 'required' on initializers (#20548)

This is actually two separate problems:
- 'override' wasn't getting added to initializers in deserialization
- 'override' was getting added when inheriting a 'required' initializer,
  even though it's normally an error to write that

Needed for parseable interfaces.

lib/Serialization/Deserialization.cpp

@@ -2222,6 +2222,15 @@ ModuleFile::getDeclChecked(DeclID DID) {
   return deserialized;
 }
 
+template <typename DERIVED>
+static bool attributeChainContains(DeclAttribute *attr) {
+  DeclAttributes tempAttrs;
+  tempAttrs.setRawAttributeChain(attr);
+  static_assert(std::is_trivially_destructible<DeclAttributes>::value,
+                "must not try to destroy the attribute chain");
+  return tempAttrs.hasAttribute<DERIVED>();
+}
+
 Expected<Decl *>
 ModuleFile::getDeclCheckedImpl(DeclID DID) {
   if (DID == 0)
@@ -2892,9 +2901,16 @@ ModuleFile::getDeclCheckedImpl(DeclID DID) {
       ctor->setStubImplementation(true);
     if (initKind.hasValue())
       ctor->setInitKind(initKind.getValue());
-    ctor->setOverriddenDecl(cast_or_null<ConstructorDecl>(overridden.get()));
     ctor->setNeedsNewVTableEntry(needsNewVTableEntry);
 
+    ctor->setOverriddenDecl(cast_or_null<ConstructorDecl>(overridden.get()));
+    if (auto *overridden = ctor->getOverriddenDecl()) {
+      if (!attributeChainContains<RequiredAttr>(DAttrs) ||
+          !overridden->isRequired()) {
+        AddAttribute(new (ctx) OverrideAttr(SourceLoc()));
+      }
+    }
+
     if (auto defaultArgumentResilienceExpansion = getActualResilienceExpansion(
             rawDefaultArgumentResilienceExpansion)) {
       ctor->setDefaultArgumentResilienceExpansion(