LLVM backend: add support for uset

[bollu]

The LLVM backend (#1837) does not implement uset, as no test case in the Lean4 test suite produces a uset. Add a test that produces a uset, and generate the right code for it. A potential test case is:

structure Point where
  x : USize
  y : UInt32

@[noinline] def Point.right (p : Point) : Point :=
  { p with x := p.x + 1 }

def main : IO Unit :=
  IO.println (Point.right ⟨0, 0⟩).x