[rustbuild] LLVM seems to not be rebuilt when submodule changes

[nagisa]

I

• Changed the submodule, commited the changes;
• Commited the submodule commit change in the rust repo;
• Built.

Rustbuild did not seem to pick up on the submodule being changed.

[petrochenkov]

LLVM rebuild is currently triggered only when contents of llvm-rebuild-trigger and llvm-finished-building stamps disagree (including llvm-finished-building being non-existent), so it's always manual.

Full checking of something in LLVM being outdated is not performed because it's expensive thing to do on every build, but the submodule state check can be performed I guess.