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

[Matt Arsenault via llvm-commits]

================
@@ -238,18 +242,40 @@ enum : int32_t {
 
 // Kernel descriptor. Must be kept backwards compatible.
 struct kernel_descriptor_t {
----------------
arsenm wrote:

I think it's useful to have the actual struct available to reference. This version doesn't need the reserved entries 

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