Warn or abort if -Zincremental is used without -Zorbit

[michaelwoerister]

Because of #34151, incremental compilation cannot work reliably without MIR-trans. We should make sure that we can't run into this accidentally.

cc @nikomatsakis

[michaelwoerister]

I added this to the milestone, since it's easy to do and can only lead to trouble once people try out the alpha.

[nikomatsakis]

Maybe we should just make -Z incremental imply -Z orbit? In any case, I just spent a bit of time debugging a case where I forgot to use -Z orbit =)