Revert "Revert "CMake: Move sphinx detection into AddSphinxTarget.cmake""

This reverts commit r302054.

Re-commit now that I have fixes for clang/lld.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302499 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed