actually use substs during const eval

[oli-obk]

The field

rust-clippy/clippy_lints/src/consts.rs

Line 219 in 2cc23a5

substs: SubstsRef<'tcx>,

is currently dead code (defaults to an empty slice always). We should make it be the current body's identity substs. Not sure if these are available in the LateContext or if we need to fetch them via the identity_substs query