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

[Greg Clayton via llvm-commits]

clayborg wrote:

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

It indeed is, but it currently can cause a crash. If DWARF was always produced with no errors then we wouldn't need this, but alas you almost alwasy run into errors when running `llvm-dwarfdump --verify --all <path>` on a lot of DWARF, so if we waited for all DWARF bugs to be fixed, we would be dead in the water most of the time.

Many optimization passes, mostly LTO in all of its forms and BOLT can try to re-write the DWARF and often introduce errors. 

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