Fix double-destruction of temporaries. (#6010)

Attach the cleanup to the `Temporary` instruction instead of to the
`TemporaryStorage` instruction. We create `TemporaryStorage`
instructions speculatively when creating an initializing expression, and
may overwrite those instructions with other instructions if it turns out
that a temporary is not required. Instead, wait until we finalize the
temporary and create a `Temporary` instruction to register the cleanup.

toolchain/check/cpp_thunk.cpp

@@ -600,7 +600,7 @@ auto PerformCppThunkCall(Context& context, SemIR::LocId loc_id,
   if (thunk_takes_return_address) {
     // Create a temporary if the caller didn't provide a return slot.
     if (!return_slot_id.has_value()) {
-      return_slot_id = AddInstWithCleanup<SemIR::TemporaryStorage>(
+      return_slot_id = AddInst<SemIR::TemporaryStorage>(
           context, loc_id, {.type_id = return_type_id});
     }