MIRI: Tried to cache DefId(_), but was already existing as AllocId(_)

[michaelwoerister]

The following crash occurs during incremental compilation of the rust-url crate:
https://travis-ci.org/rust-icci/rust-url/jobs/354013054#L1157-L1159

cc @oli-obk @eddyb

[oli-obk]

minimal repro:

pub static A: i32 = 42;
pub static B: &i32 = &A;

[michaelwoerister]

@oli-obk remarked on IRC that this might be temporarily worked around by not caching upstream constants.

[nikomatsakis]

triage: P-high

Regression.