Transition fully to NLL (instead of the migration mode currently used).

• one currently opts into this via #![feature(nll)] or -Z borrowck=mir
• It gets rid of AST-region inference errors, causing some code to be accepted that is rejected by migrate mode, and changing diagnostics in many cases.
• Potential blockers:
  • ~NLL-sound bugs that affect NLL mode without migration
    • NLL accepts higher-ranked subtype that non-NLL rejects #57642 (needs test for now)
  • Diagnostic differences
    • List: https://github.com/rust-lang/rust/issues?q=is%3Aopen+is%3Aissue+label%3ANLL-diagnostics
  • Migrate compare-mode=NLL test differences to revisions (to ease stabilization PR and meticulously check diagnostic differences)
  • Two-phase borrows (2PB) question: [NLL] prohibit "two-phase borrows" with existing borrows? #56254
  • Must have crater run showing no major crates fail in practice (crater run to estimate impact of full NLL transition #60680)
  • coercion becomes order dependent for higher-ranked functions #73154
    • Likely needs changes to the leak check
  • Investigate [NLL] Lifetimes errors in a function can mask later errors in same function #96331
    • Diagnostics-only change, should be fine