Emit fatal on defaults for generic params in binders if with nested defs

[mu001999]

Fixes #123629

Such defaults won't be lowered to hir, but the nested items would be. Then ICE will happen when later error handling walks HIR parents of inner items.

Similar to #154595

[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

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[BoxyUwU]

r? BoxyUwU
given the overlap between the two PRs

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

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