[llvm] Modify llvm-gsymutil to ignore invalid file indexes. (PR #70876)

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

> This feels more like addressing the symptom than the cause. Why is the DWARF broken in the first place?

Sure, but I think llvm-gsymutil should still continue to convert, and report.

https://github.com/llvm/llvm-project/pull/70876