[RemoteMirror] Clean up templating in Remote Mirror reflection context handling of ObjC interop

[btroller]

A bit of cleanup related to changes I made a while back for #71168.

Those changes added the method withContext() to SwiftReflectionContext, which templates over both the return type and the passed lambda. This can be simplified to just templating over the lambda and returning an 'auto', which doesn't require passing the templated return type at the call sites:

  return ContextRef->withContext<int>([&](auto *Context) {
    return Context->addImage(RemoteAddress(imageStart)).has_value();
  });

⬇️

  return ContextRef->withContext([&](auto *Context) {
    return Context->addImage(RemoteAddress(imageStart)).has_value();
  });

[btroller]

@swift-ci Please test

[mikeash]

Nice little improvement.