Deprecating uses of an enum

[HertzDevil]

#11680 allows @[Deprecated] to work on constants. The same isn't true for enums:

@[Deprecated]
enum Foo
  @[Deprecated]
  Bar
end

@[Deprecated]
module Foo2
  @[Deprecated]
  Bar2 = 1
end

Foo::Bar
Foo2::Bar2 # Warning: Deprecated Foo2::Bar2.

Foo
Foo2

Ideally, @[Deprecated] on an enum member should deprecate just that member, and @[Deprecated] on an enum type should deprecate all of its members, including the implicitly generated None and All. Using Foo or Foo2 itself should also trigger a deprecation warning as part of #11043.

The compiler completely ignores annotations on enum members, but as a side effect of #12044 this is now doable because enum members are somehow represented by Arg nodes.