[cmake] Use separate doctrees to prevent races between Sphinx instances

Use separate doctrees between different Sphinx builders in order to
prevent race condition issues due to multiple Sphinx instances accessing
the same doctree cache in parallel.

Bug: https://llvm.org/bugs/show_bug.cgi?id=23781

Differential Revision: https://reviews.llvm.org/D23755

llvm-svn: 283188