Try dumping, when a SPIR-T pass panics, its in-progress spirt::Module state.

[eddyb]

While SPIR-T passes should, in principle, never panic, it can still happen (esp. with WIP code, where e.g. assert!s or todo!()s might be overrepresented etc.), and before this PR, the partially modified SPIR-T module would be completely lost in the --dump-spirt-passes output.

And because incomplete SPIR-T mutation could result in extremely malformed modules, this PR also wraps the SPIR-T pretty-printing in std::panic::catch_unwind, removing one pass' "after" state, at a time, from the pretty-printed versions, while it still panics, and showing an extra warning (with all the pass names that couldn't be printed) if that was necessary at all.

Also, there's a minor ergonomic fix in that after_pass doesn't need to also be passed the same string as before_pass (since SpirtDumpGuard is tracking it anyway).