[ClangImporter] Prefer available enum elements over unavailable ones

[jrose-apple]

• Explanation: When importing two enum elements that have the same value, we usually mark one as an alias of the other. However, if the first case is unavailable, the compiler then produces an error message about the reference to that unavailable case. To fix this, always make sure to import an available case first; if there aren't any, just import the unavailable cases as unrelated to one another.
• Scope: Affects all imported enums, but should only change behavior for those with cases marked unavailable or platform-specific, and then only when more than one case has the same value.
• Issue: rdar://problem/30025723
• Reviewed by: @DougGregor
• Risk: Medium-low.
• Testing: Added a compiler regression test, verified that the test case in the Radar no longer fails.

[jrose-apple]

@swift-ci Please test