[LLD][COFF] use offset in archive to disambiguate archive members

Summary:
Archives can contain multiple members with the same name. This would
cause ThinLTO links to fail ("Expected at most one ThinLTO module per
bitcode file"). This change implements the same strategy we use in
the ELF linker: make the offset in the archive part of the module
name so that names are unique.

Reviewers: pcc, mehdi_amini, ruiu

Reviewed By: ruiu

Subscribers: eraman, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60549

llvm-svn: 358440