Closures in enum discriminants error when they shouldn't

[leoyvens]

This used to work (regression from 1.14 stable) but now it doesn't:

enum Functions {
    Square = (|x:i32| { }, 42).1,
}

In #50623 the error less bad and some FIXME(#50689) are left in the code with hints, but still doesn't work. Issue #50688 seems related.

[nikomatsakis]

Doesn't seem mega urgent. Kind of a gray area for promotion. Marking as medium but nominating for discussion with @eddyb

[eddyb]

The old const-eval got changed at some point to eagerly evaluate constants, but miri should be fine.
Looks like we ended up with some typeck bugs in the meanwhile.

[nikomatsakis]

Something seems wacky. Assigning to eddyb to stay on top of ;) marking as P-high for now but that might be an over-reaction.