[llvm] [gsymutil] Fix how invalid LLVM_stmt_seq is set and used (PR #164015)

[Jonas Devlieghere via llvm-commits]

https://github.com/JDevlieghere approved this pull request.

Looks fine to me, but let's give @alx32 a chance to chime in too, to confirm nothing is explicitly expecting the old offset and would break.

https://github.com/llvm/llvm-project/pull/164015