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

[Peter Rong via llvm-commits]

DataCorrupted wrote:

Thanks. @alx32  is on leave and I'm not sure whether he can get the notification (reason I popped up and started fixing things) . I'll wait until Monday to land it anyway.

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