[lld] [lld] Warn for missing needed sections by --gdb-index (PR #166626)

[via llvm-commits]

aury6623 wrote:

This edit is a solution for Issue #156732 I posted a few months ago. Having this warning would have saved me a day or two of debugging! In particular, when only some files are compiled with `-ggnu-pubnames`, but `--gdb-index` is still used, gdb can act in strange ways. E.g. in one case, gdb would only acknowledge the existence of a global type within certain scopes of the same function!

Now knowing what the issue is, I found this forum post where someone is running into a similar issue:
https://groups.google.com/g/llvm-dev/c/hxnPll-6de0
and this feature request asking for `--gdb-index` to work without `-ggnu-pubnames`:
https://github.com/llvm/llvm-project/issues/34168

which suggests that other people may find this warning to be helpful too.

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This is my first time contributing to llvm-project. I know adding new tests for a change is normally expected, but I'm not sure what test changes (if any) are expected for only an addition of a warning message, or where that test should be added. Please advise. Thanks!

https://github.com/llvm/llvm-project/pull/166626