[modules] Attach dependency listeners to the module manager once when it's

created, rather than creating and attaching a new listener each time we load a
module file (yes, the old ones were kept around too!). No functionality change
intended, but a bit more sanity.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244411 91177308-0d34-0410-b5e6-96231b3b80d8