Rollup merge of #37688 - eddyb:lazy-8, r=petrochenkov

[8/n] rustc: clean up lookup_item_type and remove TypeScheme.

_This is part of a series ([prev](https://github.com/rust-lang/rust/pull/37676) | [next]()) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments._

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* `tcx.tcache` -> `tcx.item_types`
* `TypeScheme` (grouping `Ty` and `ty::Generics`) is removed
* `tcx.item_types` entries no longer duplicated in `tcx.tables.node_types`
* `tcx.lookup_item_type(def_id).ty` -> `tcx.item_type(def_id)`
* `tcx.lookup_item_type(def_id).generics` -> `tcx.item_generics(def_id)`
* `tcx.lookup_generics(def_id)` -> `tcx.item_generics(def_id)`
* `tcx.lookup_{super_,}predicates(def_id)` -> `tcx.item_{super_,}predicates(def_id)`

diff --cc src/librustc/infer/error_reporting.rs
Simple merge

diff --cc src/librustc_metadata/decoder.rs
Simple merge

diff --cc src/librustc_metadata/encoder.rs
Simple merge

diff --cc src/librustc_save_analysis/lib.rs
Simple merge

diff --cc src/librustc_typeck/astconv.rs
Simple merge

diff --cc src/librustc_typeck/check/mod.rs
Simple merge

diff --cc src/librustc_typeck/check/regionck.rs
Simple merge