Driver: refactor SSP argument handling (NFC)
Driver: refactor SSP argument handling (NFC)

Out-of-line the SSP argument handling for the sake of readability.  Pass
along some state information to avoid re-computing the command line
flags.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@312084 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang