lint and ast_util: support Arm's as first class ast entities with node-id's.
rust-needsdrop: switch to lint infrastructure for "early drop" warnings.

I have been having some difficulty bootstrapping this (I eventually
resorted to switching to `-W warnings` in `mk/target.mk`).  Figure out
whether I can indeed just do `#[warn(early_drop)]` to sidestep the
outermost `-D warnings` or if there is something else going on here
that I do not yet get.