[utils/Xcode] Fix create-lit-site-cfg.sh to not fail if FileCheck isn't found.
[utils/Xcode] Fix create-lit-site-cfg.sh to not fail if FileCheck isn't found.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild