Serialization: Bring back shadowing but only after the filtering

Followup fix to #80009. We can still get ambiguities from colliding
decls across modules with the deserialization filtering. Bring back
calling the general lookup shadowing after the filtering. This way it
won't use filtered out decls to hide potential candidates.

rdar://148286345

lib/Serialization/Deserialization.cpp

@@ -2766,6 +2766,12 @@ giveUpFastPath:
   if (M)
     return diagnoseFatal();
 
+  if (values.size() > 1) {
+    // Apply shadowing filtering after other local filters so we don't rule out
+    // valid candidates shadowed by invalid ones.
+    removeShadowedDecls(values, baseModule);
+  }
+
   // When all is said and done, we should have a single value here to return.
   if (values.size() != 1) {
     return llvm::make_error<XRefError>("result is ambiguous", pathTrace,