introduce beginnings of polonius MIR dump

This is mostly for test purposes to show the localized constraints until
the MIR debugger is set up.

compiler/rustc_borrowck/src/lib.rs

@@ -316,6 +316,16 @@ fn do_mir_borrowck<'tcx>(
 
     mbcx.report_move_errors();
 
+    // If requested, dump polonius MIR.
+    polonius::dump_polonius_mir(
+        &infcx,
+        body,
+        &regioncx,
+        &borrow_set,
+        &localized_outlives_constraints,
+        &opt_closure_req,
+    );
+
     // For each non-user used mutable variable, check if it's been assigned from
     // a user-declared local. If so, then put that local into the used_mut set.
     // Note that this set is expected to be small - only upvars from closures