[Serialization] Allow conformances with no witness if allowing errors

Note that deserialization already handles this case when recovery is
enabled.

Author: bnbarham

lib/Serialization/Serialization.cpp

@@ -1440,7 +1440,8 @@ void Serializer::writeASTBlockEntity(
       data.push_back(addDeclRef(req));
       data.push_back(addDeclRef(witness.getDecl()));
       assert(witness.getDecl() || req->getAttrs().hasAttribute<OptionalAttr>()
-             || req->getAttrs().isUnavailable(req->getASTContext()));
+             || req->getAttrs().isUnavailable(req->getASTContext())
+             || allowCompilerErrors());
 
       // If there is no witness, we're done.
       if (!witness.getDecl()) return;

validation-test/Serialization/AllowErrors/invalid-witness.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
+
+public protocol SomeProto {
+  init(from: SomeProto)
+}
+
+struct A {}
+struct B: SomeProto {
+  let a: A
+}
+
+let thing = B(a: A())