Auto merge of #43247 - est31:master, r=alexcrichton
Tidy: allow common lang+lib features
This allows changes to the Rust language that have both library
and language components share one feature gate.
The feature gates need to be "about the same change", so that both
library and language components must either be both unstable, or
both stable, and share the tracking issue.
Removes the ugly "proc_macro" exception added by #40939.
Closes #43089