Reapply: [llvm-objcopy] [COFF] Implement --add-gnu-debuglink

This was reverted since it broke a couple buildbots. The reason
for the breakage is not yet known, but this time, the test has
got more diagnostics added, to hopefully allow figuring out
what goes wrong.

Differential Revision: https://reviews.llvm.org/D57007

llvm-svn: 351931