[Serialization] Skip deserializing invalid attributes if allowing errors

Most invalid attributes are skipped from serialization entirely, but
custom attributes don't have their invalid bit set - the particular
custom attribute (eg. a property wrapper) is requested when needed and
skipped if invalid. Those checks can't set the invalid bit since the
attribute could be a different custom attribute (eg. a result builder).

Author: bnbarham

lib/Serialization/Deserialization.cpp

@@ -4505,6 +4505,9 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
             skipAttr = true;
           } else
             return deserialized.takeError();
+        } else if (!deserialized.get() && MF.allowCompilerErrors()) {
+          // Serialized an invalid attribute, just skip it when allowing errors
+          skipAttr = true;
         } else {
           auto *TE = TypeExpr::createImplicit(deserialized.get(), ctx);
           auto custom = CustomAttr::create(ctx, SourceLoc(), TE, isImplicit);

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

@@ -0,0 +1,14 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -module-name errors -emit-module -o %t/errors.swiftmodule -experimental-allow-module-with-compiler-errors %s
+
+// Property wrappers are invalid on top level code, check allowing errors
+// does not crash
+
+@propertyWrapper
+public struct Wrapper<T> {
+  public var wrappedValue: T
+  public init() {}
+}
+@Wrapper
+public var topLevelVar: Int = 10