Enable NLL migrate mode on the 2015 edition
## What is in this PR?
* Remove the `-Zborrowck=ast` flag option from rustc.
* The default in the 2015 edition is now `-Zborrowck=migrate`.
* The 2018 edition default is unchanged: it's still `-Zborrowck=migrate`.
* Enable two-phase borrows (currently toggled via the `-Ztwo-phase-borrows` flag) on all editions.
* Remove most dead code that handled these options.
* Update tests for the above changes.
## What is *not* in this PR?
These are left for future PRs
* Use `-Zborrowck=mir` in NLL compare mode tests (#56993)
* Remove the `-Zborrowck=compare` option (#59193)
* Remove the `-Ztwo-phase-borrows` flag. It's kept, as a flag that does nothing so that perf.rlo has time to stop using it (cc @Mark-Simulacrum)
* Remove MIR typeck as its own MIR pass - it's now run by NLL.
* Enabling `-Zborrowck=mir` by default (#58781)
* Replace `allow_bind_by_move_patterns_with_guards` and `check_for_mutation_in_guard_via_ast_walk` with just using the feature gate. (#59192)
Soundness issues that are fixed by NLL will stay open until full NLL is emitting hard errors. However, these diagnostics and completeness issues can now be closed:
Closes#18330Closes#22323Closes#23591Closes#26736Closes#27487Closes#28092Closes#28970Closes#29733Closes#30104Closes#38915Closes#39908Closes#43407Closes#47524Closes#48540Closes#49073Closes#52614Closes#55085Closes#56093Closes#56496Closes#57804
cc #43234
r? @pnkfelix
cc @rust-lang/lang
cc @rust-lang/wg-compiler-nll