message: [cmake] Only set LLVM_CODESIGN_IDENTITY for repl_swift on Darwin
  This variable being set was causing cross compilations from X -> Darwin
  to attempt to codesign using xcrun. Simply opt out unless we have a
  usable xcrun.
diff: @@ -10,8 +10,12 @@ if( CMAKE_SYSTEM_NAME MATCHES "Linux" )
  set(CMAKE_INSTALL_RPATH "$ORIGIN/../lib/swift/linux:${CMAKE_INSTALL_RPATH}")
  endif()
  +# Only set LLVM_CODESIGNING_IDENTITY for building on Apple hosts for Apple
  +# targets
  +if (CMAKE_HOST_APPLE AND APPLE)
  # Override locally, so the repl is ad-hoc signed.
  set(LLVM_CODESIGNING_IDENTITY "-")
  +endif()
  # Requires system-provided Swift libs.
  set(CMAKE_OSX_DEPLOYMENT_TARGET 10.14.4)
  