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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Given this is about LLVM project's compatibility with other LLVM projects, it does feel like fixing the root cause would be good - though I don't fundamentally object to making tools more robust to unexpected inputs.

It'd be good to at least get test cases and bugs filed for the cases where this is a problem, perhaps?

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