[TypeChecker] Address review comments

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

Author: xedin

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");
     }
 

lib/Sema/TypeCheckConstraints.cpp

@@ -2006,8 +2006,7 @@ class FallbackDiagnosticListener : public ExprTypeCheckListener {
             [](const std::pair<ConstraintLocator *, ProtocolConformanceRef>
                    &conformance) -> bool {
               auto &ref = conformance.second;
-              return ref.isConcrete() && (ref.getConcrete()->isInvalid() ||
-                                          ref.getConcrete()->isIncomplete());
+              return ref.isConcrete() && ref.getConcrete()->isInvalid();
             }))
       return;