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

[via llvm-commits]

itrofimow wrote:

Hi! No worries, thanks

The problem I'm talking about clearly manifests itself in this test, for instance:
https://github.com/llvm/llvm-project/blob/b975a7b9ee6941133ccad62e36de862cd6ba5ae0/bolt/test/X86/dwarf5-gdb-index-types-lld-generated.test#L12-L20
where `CU id = 3` in line 20 is just out of range

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