allow_internal_unstable valid on non proc-macros

[jdonszelmann]

This bug was technically noticed before, in #69399. That issue was closed without fixing the issue (instead fixing another issue).

Found the bug looking at this gem of a function:

if it's a function;
look if it's a proc macro and return
if it's not a proc macro, also return.....

Anyway, I'll fix this with the attribute handling rework (#131229), I'm just opening the issue to track the fact that this is now a known bug.

@rustbot claim
@rustbot label +A-Attributes

[jdonszelmann]

@rustbot label -needs-triage