[Serialization] Skip verifying attributes when allowing errors

When allowing errors any attribute could be on any decl, so don't verify
whether an attribute can appear on a decl. Note that these attributes
aren't serialized anyway since they'll be set to invalid during
typechecking and hence skipped.

Author: bnbarham

lib/Serialization/Serialization.cpp

@@ -1992,6 +1992,8 @@ getStableSelfAccessKind(swift::SelfAccessKind MM) {
 # define DECL(KIND, PARENT)\
 LLVM_ATTRIBUTE_UNUSED \
 static void verifyAttrSerializable(const KIND ## Decl *D) {\
+  if (D->Decl::getASTContext().LangOpts.AllowModuleWithCompilerErrors)\
+    return;\
   for (auto Attr : D->getAttrs()) {\
     assert(Attr->canAppearOnDecl(D) && "attribute cannot appear on a " #KIND);\
   }\

validation-test/Serialization/AllowErrors/invalid-attr.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -module-name errors -emit-module -o %t/errors.swiftmodule -experimental-allow-module-with-compiler-errors %s
+
+// @discardableResult is not allowed on a struct, make sure we don't crash
+// when allowing errors
+
+@discardableResult
+struct SomeStruct {}