Auto merge of #82422 - petrochenkov:allunst, r=oli-obk
Auto merge of #82422 - petrochenkov:allunst, r=oli-obk

expand: Do not allocate `Lrc` for `allow_internal_unstable` list unless necessary

This allocation is done for any macro defined in the current crate, or used from a different crate.
EDIT: This also removes an `Lrc` increment from each *use* of such macro, which may be more significant.
Noticed when reviewing https://github.com/rust-lang/rust/pull/82367.
This probably doesn't matter, but let's do a perf run.

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust