[Sema] initial overhaul of isNoncopyable

This implementation has the function execute a request to scan the
inheritance clause of non-protocol nominals for a `~Copyable`. For
protocols, we look in the requirement signature.

This isn't our final state, as the GenericEnvironment needs to be
queried in general to determine of a Type is noncopyable. So for now
checking for a `~Copyable` only makes sense for Decls.

lib/AST/Decl.cpp

@@ -4838,7 +4838,7 @@ GenericParameterReferenceInfo ValueDecl::findExistentialSelfReferences(
 bool TypeDecl::isNoncopyable() const {
   return evaluateOrDefault(getASTContext().evaluator,
                            IsNoncopyableRequest{const_cast<TypeDecl *>(this)},
-                           getAttrs().hasAttribute<MoveOnlyAttr>());
+                           true); // default to true for safety
 }
 
 Type TypeDecl::getDeclaredInterfaceType() const {