Add support for building Sphinx documentation when being inside LLVM source tree and using CMake

Patch by Daniel Liew <daniel.liew@imperial.ac.uk>!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206660 91177308-0d34-0410-b5e6-96231b3b80d8
Fix sphinx-build warnings in clang docs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206661 91177308-0d34-0410-b5e6-96231b3b80d8