[TypeChecker] Address review comments

* Incomplete conformance is not necessarily going to produce a diagnostic
* Use `userFacingName` to avoid assert related to special names.

lib/Sema/CSGen.cpp

@@ -1582,7 +1582,7 @@ namespace {
       // If this is `Builtin.trigger_fallback_diagnostic()`, fail
       // without producing any diagnostics, in order to test fallback error.
       if (isTriggerFallbackDiagnosticBuiltin(expr, CS.getASTContext()))
-        return nullptr;
+        return Type();
 
       // Open a member constraint for constructor delegations on the
       // subexpr type.
@@ -3143,7 +3143,7 @@ namespace {
       if (DRE->getDecl() != Context.TheBuiltinModule)
         return false;
 
-      auto member = UDE->getName().getBaseIdentifier().str();
+      auto member = UDE->getName().getBaseName().userFacingName();
       return member.equals("trigger_fallback_diagnostic");
     }