ICE when setting a enum discriminator to an unsigned constant

[msullivan]

Trying to compile:

enum test { thing = 3u }
fn main() {
    log(error, thing as int);
    assert(thing as int == 3);
}

results in

nubs/unsigned-enum-ice.rs:1:20: 1:22 error: mismatched types: expected `int` but found `uint` (int vs uint)
nubs/unsigned-enum-ice.rs:1 enum test { thing = 3u }
                                                ^~
nubs/unsigned-enum-ice.rs:1:20: 1:22 error: expected signed integer constant
nubs/unsigned-enum-ice.rs:1 enum test { thing = 3u }
                                                ^~
error: internal compiler error: tag_variants: bad disr expr

It should just have the type errors without the ICE.

[lkuper]

It might make sense to fix this at the same time as #1417.

[catamorphism]

This appears to have been magically fixed. I added the test case.

[pnkfelix]

(not magic, by the way; see ba9c3eb )