Auto merge of #117818 - fmease:properly-reject-defaultness-on-free-consts, r=cjgillot

Properly reject `default` on free const items

Fixes #117791.

Technically speaking, this is a breaking change but I doubt it will lead to any real-world regressions (maybe in some macro-trickery crates?). Doing a crater run probably isn't worth it.

Author: bors