Support Swift in platform availability attribute

Summary: This adds support for Swift platform availability attributes. It's largely a port of the changes made to https://github.com/apple/swift-clang/ for Swift availability attributes. Specifically, https://github.com/apple/swift-clang/commit/84b5a21c31cb5b0d7d958a478bc01964939b6952 and https://github.com/apple/swift-clang/commit/e5b87f265aede41c8381094bbf54e2715c8293b0 . The implementation of attribute_availability_swift is a little different and additional tests in test/Index/availability.c were added.

Reviewers: manmanren, friss, doug.gregor, arphaman, jfb, erik.pilkington, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: aaron.ballman, ColinKinloch, jrmuizel, cfe-commits

Differential Revision: https://reviews.llvm.org/D50318

llvm-svn: 346633