[llvm] [AMDGPU] MCExpr-ify MC layer kernel descriptor (PR #80855)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > > As in, amdgpu directives instead of .set? e.g.,
> > 
> > 
> > I mean what precisely does .set do? Is it defining a new external symbol for each field? I'm worried about blowing up the size of the symbol table by N entries for every function, and how you map from one function to the fields of another function
> 
> It defines a symbol with its assigned MCExpr. Do correct me if I'm wrong, but can't we set them as temporary symbols that don't end up in the symbol table (i.e., prefix the symbols with ".L")?

This would solve the issue of the day, but doesn't solve the general external object linking case. We need some way to refer to a specific external symbol's resource uses for that 

https://github.com/llvm/llvm-project/pull/80855