[Clang][Docs] Fix man page build

This patch fixes the man page build. It currently doesn't work as
SOURCE_DIR isn't set correctly (just undefined) within the
add_sphinx_target function. This patch also moves around the creation of
targets for autogenerated rst files so that both the man page and html
build can depend upon them as before only the html build depended on
them.

Fixes #62540

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D149809