remove the description about clang & llvm in the freebsd section (the reader...

remove the description about clang & llvm in the freebsd section (the reader probably know what are llvm and clang

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190246 91177308-0d34-0410-b5e6-96231b3b80d8