Responses to feedback

* Use `isSpecificProtocol` instead of `==`
* Synthesize Encodable implementation lazily
* Don’t mark conformances as Used when they don’t need to be
* Avoid creating explicit one-element arrays
* Use lookupDirect instead of walking AST manually
* Produce diagnostic for Decodable classes whose non-Decodable superclass has a failable init
* Filter lazy vars manually since getStoredProperties() does not always do so
* Amend Decodable class diagnostic text
* Check for enum RawType errors only if the type was already validated
* Update unit tests to match changes

lib/Sema/DerivedConformanceCodingKey.cpp

@@ -408,8 +408,9 @@ static bool canSynthesizeCodingKey(TypeChecker &tc, Decl *parentDecl,
       return false;
   }
 
-  if (!enumDecl->getInherited().empty() &&
-      enumDecl->getInherited().front().isError())
+  auto inherited = enumDecl->getInherited();
+  if (!inherited.empty() && inherited.front().wasValidated() &&
+      inherited.front().isError())
     return false;
 
   // If it meets all of those requirements, we can synthesize CodingKey