Serialization: Recover from errors under loadObjCMethods

The diagnostics about unintended override of Objective-C methods
deserializes more decls than strictly necessary. Any of these could
trigger a deserialization failure if they rely on missing dependencies.
Simply ignore methods failing to deserialize instead of crashing.

We could do better here as this logic may ignore methods that are actually
colliding. Instead we could put more information in the lookup table to
avoid the need for fully deserializing the decl.

rdar://138764733

lib/Serialization/ModuleFile.cpp

@@ -712,8 +712,15 @@ void ModuleFile::loadObjCMethods(
       continue;
 
     // Deserialize the method and add it to the list.
+    // Drop methods with errors.
+    auto funcOrError = getDeclChecked(std::get<2>(result));
+    if (!funcOrError) {
+      diagnoseAndConsumeError(funcOrError.takeError());
+      continue;
+    }
+
     if (auto func = dyn_cast_or_null<AbstractFunctionDecl>(
-                      getDecl(std::get<2>(result)))) {
+                      funcOrError.get())) {
       methods.push_back(func);
     }
   }