Change NLL compare mode to borrowck=migrate.

This commit changes the NLL compare mode to pass `-Z borrowck=migrate`
rather than `-Z borrowck=nll` to better test what will be deployed. It
does not include the test output updates, as separation of these commits
makes reviewing simpler.

Author: davidtwco

src/tools/compiletest/src/runtest.rs

@@ -1810,7 +1810,7 @@ impl<'test> TestCx<'test> {
 
         match self.config.compare_mode {
             Some(CompareMode::Nll) => {
-                rustc.args(&["-Zborrowck=mir", "-Ztwo-phase-borrows"]);
+                rustc.args(&["-Zborrowck=migrate", "-Ztwo-phase-borrows"]);
             }
             Some(CompareMode::Polonius) => {
                 rustc.args(&["-Zpolonius", "-Zborrowck=mir", "-Ztwo-phase-borrows"]);