[cmake] Include project name in Sphinx doctree dir to fix race conditions

Modify add_sphinx_target() to include the project name alongside builder
in Sphinx doctree directory. This aims to avoid crashes due to race
conditions between multiple Sphinx instances running in parallel that
attempt to create or read that directory simultaneously.

This problem has originally been addressed in r283188. However, that
commit presumed that there will be only one target per builder being
run. However, r314863 introduced a second manpage target, reintroducing
the race condition.

Differential Revision: https://reviews.llvm.org/D40656

llvm-svn: 319461