[Docs] Fix Sphinx documentation in CMake check.

Summary:
- `include(AddSphinxTarget)` needs to occur before checking `SPHINX_FOUND`.
- `docs-polly-html` and `docs-polly-man` are now usable again.
- Perhaps we should build docs in the CI as well?

Differential Revision: https://reviews.llvm.org/D33386

llvm-svn: 303549