Ignore test issue-45983 in the polonius compare mode

There is no difference between the NLL and Polonius outputs, and it manually tests NLLs.

src/test/ui/borrowck/issue-45983.migrate.stderr

@@ -1,5 +1,5 @@
 error: borrowed data cannot be stored outside of its closure
-  --> $DIR/issue-45983.rs:19:27
+  --> $DIR/issue-45983.rs:20:27
    |
 LL |     let x = None;
    |         - borrowed data cannot be stored into here...

src/test/ui/borrowck/issue-45983.nll.stderr

@@ -1,5 +1,5 @@
 error[E0521]: borrowed data escapes outside of closure
-  --> $DIR/issue-45983.rs:19:18
+  --> $DIR/issue-45983.rs:20:18
    |
 LL |     let x = None;
    |         - `x` is declared here, outside of the closure body
@@ -9,7 +9,7 @@ LL |     give_any(|y| x = Some(y));
    |               `y` is a reference that is only valid in the closure body
 
 error[E0594]: cannot assign to `x`, as it is not declared as mutable
-  --> $DIR/issue-45983.rs:19:18
+  --> $DIR/issue-45983.rs:20:18
    |
 LL |     let x = None;
    |         - help: consider changing this to be mutable: `mut x`

src/test/ui/borrowck/issue-45983.rs

@@ -7,6 +7,7 @@
 // revisions, don't worry about the --compare-mode=nll on this test.
 
 // ignore-compare-mode-nll
+// ignore-compare-mode-polonius
 
 //[nll]compile-flags: -Z borrowck=mir