Auto merge of #119133 - scottmcm:assert-unchecked, r=thomcc

Add `hint::assert_unchecked`

Libs-API expressed interest, modulo bikeshedding, in rust-lang/libs-team#315 (comment)

I think that means this is good for nightly, since we can always rename it before stabilization.

Tracking issue: rust-lang/rust#119131

Author: bors