Auto merge of #64790 - Centril:rip-ast-borrowck, r=matthewjasper

Auto merge of #64790 - Centril:rip-ast-borrowck, r=matthewjasper

Rest In Peace, AST borrowck (2012-2019)

After having served us for 7 years, the AST borrow-checker is no more.

This PR starts from the commit `rm -rf librustc_ast_borrowck`, building on https://github.com/rust-lang/rust/pull/64221, and is probably best read commit by commit.

Migrate mode is not removed yet as it may be useful for NLL => polonius and it is also used for the `mutable_borrow_reservation_conflict` issue (https://github.com/rust-lang/rust/issues/59159).

r? @matthewjasper

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![ast-borrowck-rip](https://user-images.githubusercontent.com/855702/65646791-91a87600-dffc-11e9-9814-deed6b821c80.png)