[PATCH] [lld][ELF] Add explictly exported atoms and export R_*_COPY'ed atoms.

[kledzik at apple.com]

  Adding a new DefinedAtom attribute does not help because the atoms are read-only (const), so you can't change their attributes as more information is discovered during the link.

  Regarding "If we use the current approach of keeping a side table, we can no longer dump the atoms from this pass...", I thought this was done in the ELF Writer, which is only called after all Passes are done.  Also, because this is done in the Writer, the side table can be owned by the Writer instead of the ELFLinkingContext.

http://llvm-reviews.chandlerc.com/D1799