Add operator== for DefinedAtom::reference_iterator.  NFC.
Add operator== for DefinedAtom::reference_iterator.  NFC.

This will be used by a future commit which wanted to compare iterators
for equality.

Also changed operator!= to be in terms of the new operator==

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@259431 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld