[Serialization] Don't serialize invalid attributes

This isn't just an optimization; we weren't recording that the
attribute was invalid, and so it was getting /treated as valid/ when
the module was imported into a client later.

This would have caught the issue fixed by the previous commit.

Author: jrose-apple

lib/Serialization/Serialization.cpp

@@ -2186,6 +2186,12 @@ void Serializer::writeDeclAttribute(const DeclAttribute *DA) {
   if (DA->isNotSerialized())
     return;
 
+  // Ignore attributes that have been marked invalid. (This usually means
+  // type-checking removed them, but only provided a warning rather than an
+  // error.)
+  if (DA->isInvalid())
+    return;
+
   switch (DA->getKind()) {
   case DAK_RawDocComment:
   case DAK_ReferenceOwnership: // Serialized as part of the type.

test/Serialization/attr-invalid.swift

@@ -0,0 +1,14 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t/attr.swiftmodule %s -verify
+// RUN: llvm-bcanalyzer -dump %t/attr.swiftmodule | %FileCheck -check-prefix=CHECK-NON-RESILIENT %s
+// RUN: %target-swift-frontend -emit-module -o %t/attr_resilient.swiftmodule -enable-resilience -warnings-as-errors %s
+// RUN: llvm-bcanalyzer -dump %t/attr_resilient.swiftmodule | %FileCheck -check-prefix=CHECK-RESILIENT %s
+
+// These two should be checking for the same thing.
+// CHECK-RESILIENT: Frozen_DECL_ATTR
+// CHECK-NON-RESILIENT-NOT: Frozen_DECL_ATTR
+
+@_frozen // expected-warning {{@_frozen has no effect without -enable-resilience}}
+public enum SomeEnum {
+  case x
+}

test/api-digester/compare-dump.swift

@@ -1,8 +1,8 @@
 // RUN: %empty-directory(%t.mod)
 // RUN: %empty-directory(%t.sdk)
 // RUN: %empty-directory(%t.module-cache)
-// RUN: %swift -emit-module -o %t.mod/cake1.swiftmodule %S/Inputs/cake1.swift -parse-as-library -I %S/Inputs/APINotesLeft %clang-importer-sdk-nosource
-// RUN: %swift -emit-module -o %t.mod/cake2.swiftmodule %S/Inputs/cake2.swift -parse-as-library -I %S/Inputs/APINotesRight %clang-importer-sdk-nosource
+// RUN: %swift -emit-module -o %t.mod/cake1.swiftmodule %S/Inputs/cake1.swift -parse-as-library -enable-resilience -I %S/Inputs/APINotesLeft %clang-importer-sdk-nosource
+// RUN: %swift -emit-module -o %t.mod/cake2.swiftmodule %S/Inputs/cake2.swift -parse-as-library -enable-resilience -I %S/Inputs/APINotesRight %clang-importer-sdk-nosource
 // RUN: %api-digester -dump-sdk -module cake1 -o %t.dump1.json -module-cache-path %t.module-cache %clang-importer-sdk-nosource -I %t.mod -I %S/Inputs/APINotesLeft
 // RUN: %api-digester -dump-sdk -module cake2 -o %t.dump2.json -module-cache-path %t.module-cache %clang-importer-sdk-nosource -I %t.mod -I %S/Inputs/APINotesRight
 // RUN: %api-digester -diagnose-sdk -print-module --input-paths %t.dump1.json -input-paths %t.dump2.json > %t.result