Use alias in LLVM::GlobalOp lowering once LLVM IR Dialect has it

[bcardosolopes]

While working at this, I noticed that LLVM IR dialect GlobalOp does not yet support alias, so we cannot yet lower this information down. Let me file another issue for that.

For now, I'm going to emit a global but not yet set the alias, won't be much better than what we currently have but it's a first step - later on we can just set the alias when MLIR support comes.

Originally posted by @bcardosolopes in #1048 (comment)

[smeenai]

Shouldn't we file this upstream?

[bcardosolopes]

Mostly worried about tracking what we need to do in CIR once this lands, retitle to be more clear. We should file this upstream against MLIR for sure, but since @thevinster might grab this I'll let him decide if he wants to produce a PR or an issue (or both).