This commit modifies the wording of the warning for
backwards-incompatible changes in migrate mode. The warning messages are
changed to be lowercase and not include line-breaks in order to be
consistent with other compiler diagnostics.
This commit updates the test output for the updated NLL compare mode
that uses `-Z borrowck=migrate` rather than `-Z borrowck=mir`. The
previous commit changes `compiletest` and this commit only updates
`.nll.stderr` files.
