xcode_requirement: fix Swift compatability check.
xcode_requirement: fix Swift compatability check.

Ensure we're using the full macOS version so that
`MacOS::Xcode.version >= "10.14.4"` can ever be true.

vitorgalvao/brew,konqui/brew,mahori/brew,MikeMcQuaid/brew,konqui/brew,Homebrew/brew,sjackman/homebrew,Homebrew/brew,sjackman/homebrew,reitermarkus/brew,JCount/brew,vitorgalvao/brew,MikeMcQuaid/brew,JCount/brew,nandub/brew,mahori/brew,EricFromCanada/brew,claui/brew,vitorgalvao/brew,JCount/brew,reitermarkus/brew,reitermarkus/brew,EricFromCanada/brew,nandub/brew,konqui/brew,Homebrew/brew,EricFromCanada/brew,konqui/brew,MikeMcQuaid/brew,mahori/brew,sjackman/homebrew,MikeMcQuaid/brew,nandub/brew,sjackman/homebrew,claui/brew,nandub/brew,claui/brew,Homebrew/brew,JCount/brew,EricFromCanada/brew,vitorgalvao/brew,mahori/brew,reitermarkus/brew,claui/brew