[ELF] --gdb-index: use lower_bound to compute relative CU index in the object file

Summary:
This reinstates what I originally intended to do in D54361.
It removes the assumption that .debug_gnu_pubnames has increasing CuOffset.

Now we do better than gold here: when .debug_gnu_pubnames contains
multiple sets, gold would think every set has the same CU index as the
first set (incorrect).

Reviewed By: ruiu

Reviewers: ruiu, dblaikie, espindola

Subscribers: emaste, arichardson, arphaman, llvm-commits

Differential Revision: https://reviews.llvm.org/D54483

llvm-svn: 347820