[llvm] [BOLT] Fix incorrect CU-indicies in gdb-index (PR #151927)

[via llvm-commits]

itrofimow wrote:

Hi @ayermolo !
I see you are listed as a maintainer of DWARF support in BOLT, and although this is not exactly DWARF-related, you are also 
author of this change 
https://github.com/llvm/llvm-project/blob/fde9ee1ac291652718124257796efff91a2d5baf/bolt/lib/Core/GDBIndex.cpp#L127-L132
(coming from this commit https://github.com/llvm/llvm-project/commit/dcfa2ab534c8f1b4345012a02f130a0a07060bfd), which I think introduced the problem, so could you please have a look at this PR?

There's no rush for me to get this landed, but I intend to backport this change into BOLT we use, so I'd appreciate a review

https://github.com/llvm/llvm-project/pull/151927