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_function.cpp

@@ -748,7 +748,7 @@ auto HandleParseNode(Context& context, Parse::FunctionDefinitionId node_id)
       context.emitter().Emit(LocIdForDiagnostics::TokenOnly(node_id),
                              MissingReturnStatement);
     } else {
-      AddReturnCleanupBlock(context, node_id);
+      AddReturnInstWithCleanups(context, node_id);
     }
   }