To solve your issue please try to download the next to latest all command line tools and install them then write following command in the console:
sudo xcode-select --switch /Library/Developer/CommandLineTools
You can revert your Apple Clang to the original version of the tools. By writing following command in the console:
sudo xcode-select --switch /Applications/Xcode.app
Please try the homebrew's version of llvm it will work for you.
Just write following command:
brew install llvm37
After that try to set your compiler to homebrew's clang: /usr/local/bin/clang++-3.7
By this way you will be able to leave your OS X dev environment to defaults as they were
Just rename your Xcode existing in your mac as the Xcode-7.app in your /Applications folder. After that just download Xcode6.4 from below link: https://developer.apple.com/download/more/
And just install it. And you will be sorted.