Problem:
- Several files have missing includes. This results in complaints from
`clang-tidy`.
- `#ifdef` is followed by `#elif <value>` which evaluates to `0`.
Solution:
- Add missing includes.
- Change to `#elif defined(<value>)`.
MemoryManager cannot use the Singleton class because
MemoryManager::initialize is called before the global constructors
are run. That caused the Singleton to be re-initialized, causing
it to create another MemoryManager instance.
Fixes#3226
