When adding conditionally included compiler flags, dont use the CFLAGS section.
When adding conditionally included compiler flags, dont use the CFLAGS section.

- cmake/modules/SwiftSupport.cmake prepends each flag with '-Xcc' but
  if the flag evaluates to empty then an extra -Xcc is added to the
  compiler options.

- Explicitly add the option with a conditional '-Xcc' before it as well.

- This fixes the "warning: argument unused during compilation: '-Xcc'"
  seen in the error logs.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch