Auto merge of #122505 - oli-obk:visit_nested_body2, r=tmiasko

Don't walk the bodies of free constants for reachability.

follow-up to #122371

cc #119214

This avoids codegening items (e.g. functions) that are only used during const eval, but do not reach their final constant value (e.g. via function pointers).

r? `@tmiasko`

Author: bors