Change SphinxDocsBuilder to use UnifiedTreeBuilder.
As a part of the Buildbot migration to GitHub monorepo we want to consolidate the places where the source code gets checked out. UnifiedTreeBuilder provides a good abstraction for that. Once UnifiedTreeBuilder will add a support for monorepo, the SphinxDocsBuilder users should be fine.
Patch by Alex Orlov.
Differential Revision: https://reviews.llvm.org/D68955