Fix @objcImpl crash with async/sync overloads

The @objcImpl checker would accidentally dereference a null pointer when it tried to check if an async requirement could be satisfied by a non-async method. Fix that mistake.

Fixes rdar://111064481.

Author: beccadax

lib/Sema/TypeCheckDeclObjC.cpp

@@ -2950,7 +2950,7 @@ class ObjCImplementationChecker {
     if (auto func = dyn_cast<AbstractFunctionDecl>(req)) {
       auto asyncFunc = func->getAsyncAlternative();
 
-      if (auto asyncAccessor = dyn_cast<AccessorDecl>(asyncFunc))
+      if (auto asyncAccessor = dyn_cast_or_null<AccessorDecl>(asyncFunc))
         return asyncAccessor->getStorage();
 
       return asyncFunc;

test/decl/ext/Inputs/objc_implementation.h

@@ -94,6 +94,9 @@
 - (void)doSomethingAsynchronousWithCompletionHandler:(void (^ _Nonnull)(id _Nullable result, NSError * _Nullable error))completionHandler;
 - (void)doSomethingElseAsynchronousWithCompletionHandler:(void (^ _Nullable)(id _Nonnull result))completionHandler;
 - (void)doSomethingFunAndAsynchronousWithCompletionHandler:(void (^ _Nonnull)(id _Nullable result, NSError * _Nullable error))completionHandler;
+
+- (void)doSomethingOverloadedWithCompletionHandler:(void (^ _Nonnull)())completionHandler;
+- (void)doSomethingOverloaded __attribute__((__swift_attr__("@_unavailableFromAsync(message: \"Use async doSomethingOverloaded instead.\")")));
 @end
 
 @protocol PartiallyOptionalProtocol

test/decl/ext/objc_implementation.swift

@@ -312,6 +312,14 @@ protocol EmptySwiftProto {}
 
   public func doSomethingFunAndAsynchronous(completionHandler: @escaping (Any?, Error?) -> Void) {
   }
+
+  @available(SwiftStdlib 5.1, *)
+  @objc(doSomethingOverloadedWithCompletionHandler:)
+  public func doSomethingOverloaded() async {}
+
+  @available(*, noasync)
+  @objc(doSomethingOverloaded)
+  public func doSomethingOverloaded() {}
 }
 
 @_objcImplementation(Conformance) extension ObjCClass {