Use a multimap rather than a map for holding the list of copies to insert, so we...

Use a multimap rather than a map for holding the list of copies to insert, so we don't lose copies when two of them have
the same source.  I don't know what I was thinking when I wrote this originally.
Note: There's probably a more efficient way to do this, but I need to think about it some more, and about what determinism
guarantees need to be present.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56964 91177308-0d34-0410-b5e6-96231b3b80d8