Improve toolchain identification on macOS

[jld01]

The LLVM toolchain supplied by recent versions of the XCode Command Line Tools package is not identified by CDT, so the toolchain is not presented for selection in the MBS project wizard.

To improve the out-of-box experience for new CDT users on macOS, it would also be useful to automatically locate tools provided by the Homebrew package manager on macOS, in a similar manner to the way we support the MSYS2 package manager on Windows.

[jld01]

I am closing this issue. CDT now detects the version of clang provided by Xcode Command Line Tools and will locate Homebrew installations.