Make Scope private to rustc_mir::build::scope::stack

The logic for finding the correct unwind scopes is going to become more
complicated, so we want to limit the amount of code that can directly
access the fields of `Scope`.

Author: matthewjasper

src/librustc_mir/build/matches/mod.rs

@@ -228,7 +228,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         };
 
         // Step 5. Create everything else: the guards and the arms.
-        let match_scope = self.scopes.topmost();
+        let match_scope = self.topmost_scope();
 
         let arm_end_blocks: Vec<_> = arm_candidates.into_iter().map(|(arm, mut candidates)| {
             let arm_source_info = self.source_info(arm.span);