[llvm] Modify llvm-gsymutil to ignore invalid file indexes (PR #71431)

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

> > DWARF produced by LTO and BOLT can sometimes be broken where file indexes are beyond the end of the line table's file list in the prologue.
> 
> Bugs are always possible, but I'm pretty sure LTO would haev a hard time producing this sort of bug... - maybe BOLT's the root cause? In any case, as noted previously, some kind of bug reports would be appreciated.

For once it is not. :D

So I dug into it internally a bit. Looks like issue was with llvm-12, and goes away with llvm-15 (version we are migrating to currently).

https://github.com/llvm/llvm-project/pull/71431