SafeStack: Disable Darwin support
SafeStack: Disable Darwin support

Summary:
Darwin support does not appear to be used as evidenced by the fact that
the runtime has never supported non-trivial programs.

Reviewers: pcc, kubamracek

Reviewed By: pcc

Subscribers: cfe-commits, kcc

Differential Revision: https://reviews.llvm.org/D50724

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@339720 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit d203e14677d0ca95bf0f0009fe91794e2c8261aa)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang