[modules] Add first-draft module maps for Clang. These don't cover everything
yet, but they're enough for a clean bootstrap (with a few local patches that
are yet to be committed).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210726 91177308-0d34-0410-b5e6-96231b3b80d8