Add notifier hooks to symbol table.

This is the one interesting aspect from:
   http://reviews.llvm.org/D4965

These hooks are useful for flavor specific processing, such as recording that
a DefinedAtom replaced a weak SharedLibraryAtom.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@216122 91177308-0d34-0410-b5e6-96231b3b80d8