compiletest: known-bug should take an issue number

[jackh726]

Basically, instead of // known-bug, do // known-bug #00000 or maybe // known-bug chalk#00?

From: https://rust-lang.zulipchat.com/#narrow/stream/326866-t-types.2Fnominated/topic/.2398095.3A.20NLL.3A.20unsound.20verification.20of.20higher.20ranked.20outlives.E2.80.A6/near/287258738

[davidkna]

@rustbot claim