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

[David Blaikie via llvm-commits]

dwblaikie wrote:

(if making the tool more robust also helps isolate/identify the root causes, that's a different situation than only making the tool able to progress in the face of these problems)

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