llvm: Do not install xcode-toolchain for Linux
llvm: Do not install xcode-toolchain for Linux

Linuxbrew/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,maxim-belkin/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core