[Serialization] Recover from deserialization failures in ModuleFile::lookupObjCMethods

This was reported by deserialization safety in:

Interpreter/SDK/Foundation_NSString.swift

Author: xymus

lib/Serialization/ModuleFile.cpp

@@ -885,8 +885,13 @@ void ModuleFile::lookupObjCMethods(
   auto found = *known;
   for (const auto &result : found) {
     // Deserialize the method and add it to the list.
-    if (auto func = dyn_cast_or_null<AbstractFunctionDecl>(
-                      getDecl(std::get<2>(result))))
+    auto declOrError = getDeclChecked(std::get<2>(result));
+    if (!declOrError) {
+        consumeError(declOrError.takeError());
+        continue;
+    }
+
+    if (auto func = dyn_cast_or_null<AbstractFunctionDecl>(declOrError.get()))
       results.push_back(func);
   }
 }

test/Serialization/Safety/lookupObjCMethods.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -typecheck -verify %s \
+// RUN:   -enable-deserialization-safety \
+// RUN:   -Xllvm -debug-only=Serialization 2>&1 | %FileCheck %s
+
+// REQUIRES: objc_interop
+// REQUIRES: asserts
+
+import Foundation
+
+// Fails at reading __SwiftNativeNSEnumerator.init() on macOS.
+NSString.instancesRespond(to: "init") // expected-warning {{use of string literal for Objective-C selectors is deprecated; use '#selector' instead}}
+// CHECK: Skipping unsafe deserialization: 'init()'