Add tests for overlapping explicitly dropped locals in generators

[tmandry]

#61922 allows us to overlap x and y in this example:

fn gen() {
  let x = [0u8; 1024];
  yield;
  drop(x);
  let y = [0u8;1024];
  yield;
  drop(y);
}

Come to think of it, this is a pretty important behavior that I should have added an explicit test for.