Revert "'@available' without an OS is ignored on extensions" warning #16298
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This is still useful for the feature where deprecated declarations are allowed to be used within a context that's marked deprecated. There's probably a better balance to be found here, because marking the extension deprecated still does not mark all the members deprecated, but for now it still has an effect, and we shouldn't produce a warning for legitimately making use of that effect.
The warning was added in #15213, mostly as an opportunity "fix".
SR-7577 / rdar://problem/39867773