[LLD][COFF] Add index to disambiguate archive members when using -wholearchive

Patch by Markus Böck.

PR42951: When linking an archive with members that have the same name linking
fails when using the -wholearchive option. This patch passes the index
of the member in the archive to the offset parameter to disambiguate the
member.

Differential Revision: https://reviews.llvm.org/D66239

llvm-svn: 371509