[ELF] - Allow multiple comdats when producing relocatable output.

Previously LLD would fail for case when there are multiple comdats and -r.

That happened because it merged all ".group" (SHT_GROUP) sections into single
output, producing broken result. Such sections may have similar name, alignment and flags
and other properties. We need to produce separate output section for each such input one.

Differential revision: https://reviews.llvm.org/D33643

llvm-svn: 304769