[llvm] [Object] Provide operator< for ELFSymbolRef (PR #89861)

[Amir Ayupov via llvm-commits]

aaupov wrote:

> I've not really looked myself, but are there instances where you actually have two ELFSymbolRef objects that are compared (using `<`)? If not, this code is dead, I believe. 

I'm going to use it in FILE symbol lookup using upper_bound on ELFSymbolRefs here, instead of using DRI.d.b (symbol index) directly: https://github.com/llvm/llvm-project/pull/89648


> Also, is there a reason you provide `<` but not `==`? I note that `DataRefImpl` has both.

DataRefImpl's `operator==` is good enough for comparison.

https://github.com/llvm/llvm-project/pull/89861