Remove '-Xclang' and '-mllvm' entries from Clang man page.  The later is only available in debug builds and the former is not supposed to be used by end-users.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127914 91177308-0d34-0410-b5e6-96231b3b80d8