Find better solution for marking function aliases as definitions

[Jezurko]

Prerequisites

• I have read the documentation and the proposed feature is not implemented.

Description

The current implement of isDeclaration() in core dialect is quite wonky, as it looks for a hl.alias named attribute, to detect that a function is an alias. This breaks the abstraction of core::FuncBaseOp being a generic function operation base (although so far no other dialect has an attribute for function aliasing).