[docs] Introduce ExternalProjectsUsingClang.rst

A couple concrete examples are sure to be a win. If you know of any
other external projects using Clang, please let me know!

Patch by Laszlo Nagy! (with a title tweak by me)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175529 91177308-0d34-0410-b5e6-96231b3b80d8