clang-cl: Add /FS as an ignored flag
clang-cl: Add /FS as an ignored flag

CMake-generated Makefiles include this cl flag which can be safely ignored
until we get PDB server support.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@194165 91177308-0d34-0410-b5e6-96231b3b80d8

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