[[llvm] Clearly document the OS X requirement for full XCode installation

The llvm/clang++ compiler used and provided by mason needs a full XCode installation available. Otherwise you'll hit a warning like:

clang-5.0: warning: no such sysroot directory: '/Library/Developer/CommandLineTools/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk' [-Wmissing-sysroot]

And an error when the program tries to use something from the C standard library. For example:

fatal error: 'stdlib.h' file not found
#include <stdlib.h>
         ^~~~~~~~~~
1 error generated.

But this error will depend on what of the C standard library is being requested.

The way to solve this is for OS X users to install a full version of XCode.

This reliance on a full version of XCode is due to this line in our compiler config: https://github.com/mapbox/mason/blob/6aa9591a336ac71bfa1974185b6ee784a9b6033a/scripts/llvm/base/common.sh#L230. It is possible we could try to get this working against the Command line tools install of C headers as well, but this is not something I've tried.

Start at scripts/llvm/base/common.sh around line 230 to confirm why the compiler requires a full XCode installation rather than Command Line Tools. Document that OS X prerequisite, including the reported missing-sysroot and stdlib.h errors, in the project's appropriate documentation location. Done means users can find and understand the full XCode requirement before using mason's llvm/clang++ compiler.