Support revisions on global commands

[RalfJung]

From what I could tell, revisions are currently only supported on error patterns. Would be nice to also support them on global commands, so that one can do things like

//@revisions: isolated free
//@only-target-linux
//[free]@compile-flags: -Zmiri-disable-isolation

FWIW I would have probably picked this syntax

//@revisions: isolated free
//@only-target-linux
//@[free]compile-flags: -Zmiri-disable-isolation

but that would be inconsistent with //[fn_ptr]~^ ERROR: pattern... maybe we should change that to //~[fn_ptr]^ ERROR: pattern, too, so that the first character after the // always indicates what this is?

OTOH, that might create parsing ambiguities if there is no ERROR level, and //[revision] is also consistent, so... whatever.

[oli-obk]

We could require //@[revision]~ foo if you want to use revisions

[RalfJung]

That feels odd, I associate @ with global commands and ~ with patterns.

[oli-obk]

considering square brackets in diagnostics are rare and you could always use a longer or regex pattern in that case (and get an error if that [revision] is not actually a revision), I think we should just use the possibly ambigous case

[RalfJung]

I forgot, do we even still have the no-error-level syntax?

[oli-obk]

oh haha, no I scrapped that and added a global min-level flag to get the "don't require annotaitons for all notes" behaviour. Ok no ambiguity, woo

[RalfJung]

Right so the only question is if we want to incur all that churn for when/if rustc itself uses this crate.

[oli-obk]

Right so the only question is if we want to incur all that churn for when/if rustc itself uses this crate.

I can incrementally move the rustc test suite to use the same syntax to keep the final diff small