view-types: HIR lowering

[scrabsha]

This PR implements the AST -> HIR lowering of view types and a few checks.

This PR is over #156016, the first new commit is f8266f0.

r? oli-obk

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

HIR ty lowering was modified

cc @fmease

rustfmt is developed in its own repository. If possible, consider making this change to rust-lang/rustfmt instead.

cc @rust-lang/rustfmt