[modules] Change module manager visitation order to be a bit more stable when

more modules are added: visit modules depth-first rather than breadth-first.
The visitation is still (approximately) oldest-to-newest, and still guarantees
that a module is visited before anything it imports, so modules that are
imported by others sometimes need to jump to a later position in the visitation
order when more modules are loaded, but independent module trees don't
interfere with each other any more.

llvm-svn: 242863