[cleanup] Remove unused default argument and tidy up.

The RequestingModule argument was unused and always its default value of
nullptr.

Also move a declaration closer to its use, and range-for'ify.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239453 91177308-0d34-0410-b5e6-96231b3b80d8