doctor: check that Xcode prefix actually exists
doctor: check that Xcode prefix actually exists

Closes Homebrew/homebrew#14373.

Linuxbrew/brew,Homebrew/brew,sjackman/homebrew,sjackman/homebrew,EricFromCanada/brew,nandub/brew,zmwangx/brew,konqui/brew,palxex/brew,DomT4/brew,reitermarkus/brew,staticfloat/brew,tonyg/homebrew,sjackman/homebrew,toonetown/homebrew,mgrimes/brew,claui/brew,MikeMcQuaid/brew,bfontaine/brew,staticfloat/brew,rwhogg/brew,vitorgalvao/brew,Linuxbrew/brew,rwhogg/brew,mistydemeo/homebrew,nandub/brew,amar-laksh/brew_sudo,mahori/brew,hanxue/linuxbrew,toonetown/homebrew,pseudocody/brew,Homebrew/brew,zmwangx/brew,claui/brew,Homebrew/brew,bfontaine/brew,DomT4/brew,hanxue/linuxbrew,AnastasiaSulyagina/brew,MikeMcQuaid/brew,maxim-belkin/brew,EricFromCanada/brew,pseudocody/brew,maxim-belkin/brew,MikeMcQuaid/brew,bfontaine/brew,ilovezfs/brew,reitermarkus/brew,toonetown/homebrew,aw1621107/brew,mistydemeo/homebrew,staticfloat/brew,gregory-nisbet/brew,mgrimes/brew,amar-laksh/brew_sudo,mistydemeo/homebrew,muellermartin/dist,Homebrew/brew,nandub/brew,mahori/brew,reitermarkus/brew,MikeMcQuaid/brew,maxim-belkin/brew,konqui/brew,gregory-nisbet/brew,vitorgalvao/brew,JCount/brew,DomT4/brew,konqui/brew,ilovezfs/brew,mahori/brew,gordonmcshane/homebrew,mahori/brew,vitorgalvao/brew,Linuxbrew/brew,alyssais/brew,mgrimes/brew,aw1621107/brew,reelsense/brew,palxex/brew,claui/brew,pseudocody/brew,rwhogg/brew,gregory-nisbet/brew,Linuxbrew/brew,gordonmcshane/homebrew,jmsundar/brew,vitorgalvao/brew,JCount/brew,amar-laksh/brew_sudo,gordonmcshane/homebrew,sjackman/homebrew,ilovezfs/brew,zmwangx/brew,aw1621107/brew,tonyg/homebrew,DomT4/brew,jmsundar/brew,reelsense/brew,reitermarkus/brew,hanxue/linuxbrew,AnastasiaSulyagina/brew,nandub/brew,JCount/brew,reelsense/brew,alyssais/brew,alyssais/brew,AnastasiaSulyagina/brew,jmsundar/brew,JCount/brew,muellermartin/dist,EricFromCanada/brew,konqui/brew,claui/brew,palxex/brew,tonyg/homebrew,muellermartin/dist,EricFromCanada/brew