infinite_loop: do not defensively use saturating_sub()

Using `saturating_sub()` here in code which cannot fail brings a false
sense of security. If for any reason a logic error was introduced and
caused `self.loop_depth` to reach 0 before being decremented, using
`saturating_sub(1)` would silently mask the programming error instead of
panicking loudly as it should (at least in dev profile).

clippy_lints/src/loops/infinite_loop.rs

@@ -112,7 +112,7 @@ impl<'hir> Visitor<'hir> for LoopVisitor<'hir, '_> {
             ExprKind::Loop(..) => {
                 self.loop_depth += 1;
                 walk_expr(self, ex);
-                self.loop_depth = self.loop_depth.saturating_sub(1);
+                self.loop_depth -= 1;
             },
             _ => {
                 // Calls to a function that never return