[Macros] Diagnose top-level expansion of undefined freestanding macro

The root problem is that `ResolveMacroRequest` was shortcuting after calling `lookupMacros`. It shouldn't need to call `lookupMacros` at all, but should go straight to CSGen. It's fixed by deleting 4 lines of code.

rdar://108280416

Author: rxwei

lib/Sema/TypeCheckMacros.cpp

@@ -1483,10 +1483,6 @@ ResolveMacroRequest::evaluate(Evaluator &evaluator,
 
   auto &ctx = dc->getASTContext();
   auto roles = macroRef.getMacroRoles();
-  auto foundMacros = TypeChecker::lookupMacros(
-      dc, macroRef.getMacroName(), SourceLoc(), roles);
-  if (foundMacros.empty())
-    return ConcreteDeclRef();
 
   // If we already have a MacroExpansionExpr, use that. Otherwise,
   // create one.

test/Macros/macros_diagnostics.swift

@@ -185,3 +185,7 @@ struct MyStruct<T: MyProto> {
 
 @freestanding(expression) macro myMacro<T : MyProto>(_ value: MyStruct<T>) -> MyStruct<T> = #externalMacro(module: "A", type: "B")
 // expected-warning@-1{{external macro implementation type}}
+
+#undefinedMacro { definitelyNotDefined }
+// expected-error@-1{{cannot find 'definitelyNotDefined' in scope}}
+// expected-error@-2{{no macro named 'undefinedMacro'}}

test/Macros/macros_library_mode_diagnostics.swift

@@ -0,0 +1,13 @@
+// REQUIRES: swift_swift_parser
+
+// RUN: %target-typecheck-verify-swift -swift-version 5 -enable-experimental-feature FreestandingMacros -parse-as-library -module-name MacrosTest
+
+// We need this test because top-level freestanding macro expansions are parsed
+// differently in library mode.
+
+#undefinedMacro1
+// expected-error@-1{{no macro named 'undefinedMacro1'}}
+
+#undefinedMacro2 { definitelyNotDefined }
+// expected-error@-1{{no macro named 'undefinedMacro2'}}
+// expected-error@-2{{cannot find 'definitelyNotDefined' in scope}}