We used to allow @objc enums on Linux, where Objective-C interop is disabled,
because the check first looked for an import of the Foundation module
(which could be the non-Objective-C corelibs-foundation) before checking
if Objective-C interop was enabled.
Since @objc enums don't actually depend on Foundation or Objective-C and
only have a different layout in IRGen, let's bring back and formalize the
old behavior.
Fixes https://bugs.swift.org/browse/SR-14548 / rdar://problem/77325078.
