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

[kledzik at apple.com]

  Can you explain the "explicitly exported" concept.  Will it be needed elsewhere?   It sounds like this is logically an attribute of an atom, but it does not come from the object file.  Instead it is an attribute that came about during the link.  There are potentially other attributes like that.  For instance, liveness (used during dead stripping), or canBeNullAtRuntime (copied from Undefined atom to SharedLibraryAtom).   We should have a general solution for this.  (In ld64 I wound up adding these attributes to the base Atom class and const casting in the handful of places in the linker I needed to "set" them).

  Also, it took some head scratching to figure out what ObjectAtom was.  The term "object" is way overloaded.  I think it is for data atoms that came from STT_OBJECT symbols...

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