the test(s) from issue 25579 should be ported to NLL run-pass tests

[pnkfelix]

The work on reviewing the diagnostic differences between AST-borrowck and NLL leads me to conclude that NLL is (probably) correctly accepting the regression test we added for #25579

Namely this:

rust/src/test/ui/issues/issue-25579.rs

Lines 16 to 28 in 653da4f

	enum Sexpression {
	Num(()),
	Cons(&'static mut Sexpression)
	}
	fn causes_error_in_ast(mut l: &mut Sexpression) {
	loop { match l {
	&mut Sexpression::Num(ref mut n) => {},
	&mut Sexpression::Cons(ref mut expr) => { //[ast]~ ERROR [E0499]
	l = &mut **expr; //[ast]~ ERROR [E0506]
	}
	}}
	}

We should make the original test be AST-borrowck only (via -Z borrowck=ast) and make a new version of the test (and perhaps the original code from the description on #25579) that is NLL-only and goes in run-pass.

[pnkfelix]

This bug is so old that NLL migrate mode has already been enabled on all editions (PR #59114), and that PR revised the linked test to be // compile-pass.

Closing as fixed.