Remove llvmc and assorted build machinery for it.

Remove llvmc and assorted build machinery for it.

The problems that llvmc solved have largely been subsumed with the
tasks that the clang driver can accomplish, but llvmc lacks flexibility
and depends too heavily on the EOL'd llvm-gcc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140093 91177308-0d34-0410-b5e6-96231b3b80d8