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

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

It might not be llvm tools... 
I guess fundamental question what should happen when issues are discovered? Should one or X dies that are not quite correct cause gigs of debug information not be available for downstream consumers of llvm-gsymutil output.

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