[di] Refactor out the handling of DelegatingInit related code out of ElementCollector into its own class.

There was no real code sharing going on here and instead due to the size of
ElementCollector made it difficult to ascertain without reading the code that
the two code paths are completely separate.

This is a NFC change internal to DIMemoryUseCollector that is not visible
outside of DI.

rdar://31521023

Author: gottesmm