remove or encapsulate the remaining non-query data in tcx
I wound up removing the existing cache around inhabitedness since it didn't seem to be adding much value. I reworked const rvalue promotion, but not that much (i.e., I did not split the computation into bits, as @eddyb had tossed out as a suggestion). But it's now demand driven, at least.
cc @michaelwoerister -- see the `forbid_reads` change in last commit
r? @eddyb -- since the trickiest of this PR is the work on const rvalue promotion
cc #44137
7a4f39453c