[Xcode] Fix header search build settings.
[Xcode] Fix header search build settings.

 - ALWAYS_SEARCH_USER_PATHS=YES is the legacy setting, we don't want to use
   it. Instead just put the extra system header search in the one target that
   needs it (for LLVM's config.h include).

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