[llvm] [llvm-gsymutil] Address merged-funcs test non-determinism (PR #122308)

[via llvm-commits]

alx32 wrote:

> there still seems to be some intermittent failures on EXPENSIVE_CHECKS builds - are you relying on any sorting? llvm::sort pre-shuffles the data to check for non-deterministic bugs.

Thanks will look into - I am relying on std::stable_sort - no `llvm::sort` as of now. I will look into. 

https://github.com/llvm/llvm-project/pull/122308