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/function.cpp

@@ -469,6 +469,9 @@ auto StartFunctionDefinition(Context& context, SemIR::InstId decl_id,
 auto FinishFunctionDefinition(Context& context, SemIR::FunctionId function_id)
     -> void {
   context.inst_block_stack().Pop();
+  // Any cleanups for a function will have been handled when emitting `return`s.
+  context.scope_stack().DiscardCleanupsSince(
+      context.scope_stack().function_cleanup_scope_depth());
   context.scope_stack().Pop(/*check_unused=*/true);
 
   auto observe_block_id =