Auto merge of #55134 - davidtwco:issue-55118, r=pnkfelix

NLL: change compare-mode=nll to use borrowck=migrate

Fixes #55118.

This PR is split into two parts:

The first commit is a minor change that fixes a flaw in the existing `borrowck=migrate` implementation whereby a lint that was promoted to an error in the AST borrow checker would result in the same lint from the NLL borrow checker being downgraded to a warning in migrate mode. This PR fixes this by ensuring lints are exempt from buffering in the NLL borrow checker.

The second commit updates `compiletest` to make the NLL compare mode use `-Z borrowck=migrate` rather than `-Z borrowck=mir`. The third commit shows all the test output changes that result from this.

r? @pnkfelix

diff --cc src/librustc_mir/borrow_check/mod.rs
Simple merge