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

[Shankar Kalpathi Easwaran]

  I agree to Michaels point on that we would need to set interposable depending on what we plan to write to the output file, and this would be wrong.

  For executables, the GNU/ELF linkers only export what is actually needed by shared libraries.
  For shared libraries, the GNU linkers export everything.

  What we want to export from a linker is a derived attribute in the atom for ELF, and we dont have a method in the Atom to contain derived attributes.

  May be this is the first one ?

  Nick ?

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