[CIR] [RFC] Add AliasOp

[badumbatish]

Add a dedicated Alias op better represent aliases in ClangIR. Currently with emitAliasForGlobal in clang/lib/CIR/CodeGen/CIRGenModule.cpp, we're representing aliases with a Function Op, which couples FuncOp creation with AliasOp creation.

I'm still new to the project but it seems from the consensus in meeting, this will be needed

[bcardosolopes]

The overall idea sounds good to me, thanks!