[ELF] -r: rewrite SHT_GROUP content if some members are combined or discarded

* If two group members are combined, we should leave just one index in the SHT_GROUP content.
* If a group member is discarded (/DISCARD/ or upcoming -r --gc-sections combination),
  we should drop its index in the SHT_GROUP content. LLD currently crashes (`getOutputSection()` is null).

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D84129