Fix #15129

[ghost]

Add support for unit literals to const_eval.

[ghost]

It's unfortunate this pattern matching refactor had led to so many regressions but at least we're building up test coverage the hard way. Hopefully after all the changes I'd like to make, the whole thing will be in a much better shape!

[ghost]

I've now added a test for this as well:

static x: [uint, ..()] = [];
fn main() {
}

which happily compiled in master.

[alexcrichton]

Looks like this needs a rebase

[ghost]

@alexcrichton Done.