Add debugging flag to assert state of incremental cache

[Aaron1011]

In PR #85702, I added a test that relies on the incremental cache still being saved if a linker error occurs. It would be good for the test to assert that this assumption is actually true. We could add a new flag -Z assert-incr-state=<state>, where <state> can be:

• none - we didn't find any previous session dirs whatsoever
• mismatch - we found an existing session dir, but did not load it to do a command-line arg hash mismatch.
• loaded - we found and loaded an existing session dir. Note that this says nothing about whether any particular query will be found to be red or green.

[Mark-Simulacrum]

It'd be good to add these flags to rustc-perf's incr tests once this flag is implemented, I think, FWIW.

[pierwill]

@rustbot claim