[llvm] [ELFDebugObjectPlugin] Gracefully handle missing section (PR #172622)

[Lang Hames via llvm-commits]

https://github.com/lhames approved this pull request.

LGTM as an interim solution to get the test working. 

@nikic -- Do you have access to a ppc64 system to test whether debugging works with this change? (I suspect it does, but have no way to check right now)

In an ideal world we'd identify sections whose address is safe to null out, maybe by parsing the DWARF to identify which sections are used. I don't think we need to do that yet, but it's probably worth adding a TODO for.

https://github.com/llvm/llvm-project/pull/172622