[ELF] - Do not allow -r to eat comdats.

This is PR33052, "Bug 33052 - -r eats comdats ".

To fix it I stop removing group section from out when -r is given
and fixing SHT_GROUP content when writing it just like we do some
other fixup, e.g. for Rel[a]. (it needs fix for section indices that
are in group).

Differential revision: https://reviews.llvm.org/D33485

llvm-svn: 304140