This computes the var_values used by the mir_borrowck_implied_outlives_bounds query.
The old solver canonicalization does not replace early and late bound parameters,
so the only var_values we need are external regions from the signature of the nested
body as we don’t have a shared representation between this query and MIR borrowck.
Computes the implied bounds for body_def_id. This is a separate query
as it must not reveal the hidden type of opaques defined by body_def_id
for typeck roots.
This replaces all external regions in the signature of the current item with
a unique placeholder to collect its implied bounds. This mirrors the way MIR
borrowck replaces all of them with unique NLL vars.