MIR borrowck: spurious error with ref pattern in argument

[arielb1]

e.g. (or src/test/run-pass/issue-8860.rs)

fn main() {}
fn foo(ref x: Box<()>) {}

The problem is that region inference infers the borrow of x to ReEmpty (because it's live nowhere), which doesn't have an EndRegion. NLL mode (-Z nll) is not affected by this particular pathology, but I think this should still be fixed.