Block style autodetection should take upvars into account

[marijnh]

This is a slight variation on #1467:

fn main() {
    let x = 3;
    fn blah(_a: fn()) {}
    blah({|| log(error, x); });
}

Compiling that crashes when trans tries to resolve the upvar for x inside the block. I guess typeck should refuse to convert something that has upvars into a bare function. (Or, if we end up dumping bare functions, this would be moot too.)

[jruderman]

'Assertion cx.fcx.llupvars.contains_key(did.node) failed', ../src/comp/middle/trans.rs:2698

[nikomatsakis]

Yes, when I saw #1467 I thought to myself: "I bet block inference gets this wrong." The only reason I didn't fix it yet is that I was thinking of trying to dump bare fn types.