Rollup merge of #65583 - eddyb:more-query-like-cross-crate-tables, r=michaelwoerister

rustc_metadata: use a table for super_predicates, fn_sig, impl_trait_ref.

This is an attempt at a part of #65407, i.e. moving parts of cross-crate "metadata" into tables that match queries more closely.

Three new tables should be enough to see some perf/metadata size changes.
(need to do something similar to https://github.com/rust-lang/rust/pull/59953#issuecomment-542521919)

There are other bits of data that could be made into tables, but they can be more compact so the impact would likely be not as bad, and they're also more work to set up.