[modules] Add module maps for LLVM. These are not quite ready for prime-time
yet, but only a few more Clang patches need to land. (I have 'ninja check'
passing locally.)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209269 91177308-0d34-0410-b5e6-96231b3b80d8