Revert "[Package CMO] Remove workaround for skipped types in serializability pass"

This reverts commit 41bb2e0.

Author: elsh

lib/SILOptimizer/IPO/CrossModuleOptimization.cpp

@@ -938,6 +938,21 @@ void CrossModuleOptimization::makeDeclUsableFromInline(ValueDecl *decl) {
   if (decl->getEffectiveAccess() >= AccessLevel::Package)
     return;
 
+  // FIXME: rdar://130456707
+  // Currently not all types are visited in canSerialize* calls, sometimes
+  // resulting in an internal type getting @usableFromInline, which is
+  // incorrect.
+  // For example, for `let q = P() as? Q`, where Q is an internal class
+  // inherting a public class P, Q is not visited in the canSerialize*
+  // checks, thus resulting in `@usableFromInline class Q`; this is not
+  // the intended behavior in the conservative mode as it modifies AST.
+  //
+  // To properly fix, instruction visitor needs to be refactored to do
+  // both the "canSerialize" check (that visits all types) and serialize
+  // or update visibility (modify AST in non-conservative modes). 
+  if (isPackageCMOEnabled(M.getSwiftModule()))
+    return;
+
   // We must not modify decls which are defined in other modules.
   if (M.getSwiftModule() != decl->getDeclContext()->getParentModule())
     return;