[lld-link] better error message when failing to open archive members

Summary:
The message "could not get the buffer for the member defining symbol"
now also contains the name of the archive and the name of the archive
member that we tried to open.

Reviewers: ruiu

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57974

llvm-svn: 353572