This is the idiomatic way to declare type aliases in modern C++.
Flagged by Sonar Cloud as a "Code Smell", but I happen to agree
with this particular one. :^)
The LOCK_DEBUG conditional code is pretty ugly for a feature that we
only use rarely. We can remove a significant amount of this code by
utilizing a zero sized fake type when not building in LOCK_DEBUG mode.
This lets us keep the same API, but just let the compiler optimize it
away when don't actually care about the location the caller came from.
