Destroy locals at the end of blocks, not only on return (#7448)

Destroy local variables and temporaries at each `}`, and when branching
with `break` and `continue`. In `for` statements, destroy loop variables
along with anything created within the loop at the end of each loop
iteration, and destroy the cursor and range object when the loop
terminates.

Assisted-by: Gemini via Antigravity

---------

Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/check/handle_codeblock.cpp

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include "toolchain/check/context.h"
+#include "toolchain/check/control_flow.h"
 #include "toolchain/check/handle.h"
 #include "toolchain/check/unused.h"
 
@@ -11,11 +12,12 @@ namespace Carbon::Check {
 auto HandleParseNode(Context& context, Parse::CodeBlockStartId node_id)
     -> bool {
   context.node_stack().Push(node_id);
-  context.scope_stack().PushForSameRegion();
+  context.scope_stack().PushForSameRegion(ScopeStack::CleanupScopeKind::Owned);
   return true;
 }
 
 auto HandleParseNode(Context& context, Parse::CodeBlockId /*node_id*/) -> bool {
+  AddAndDiscardCleanups(context);
   context.scope_stack().Pop(/*check_unused=*/true);
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::CodeBlockStart>();