This warning is produced if you annotate an enum with `@frozen` but
build without library evolution enabled. The problem is that if you
only build with library evolution enabled for release builds, this
leaves you with a warning for all debug builds. The downside of this
change is that if you never build with library evolution, you may have
this unnecessary annotation without noticing.
This isn't just an optimization; we weren't recording that the
attribute was invalid, and so it was getting /treated as valid/ when
the module was imported into a client later.
This would have caught the issue fixed by the previous commit.
