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

[Matt Arsenault via llvm-commits]

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

I believe the intent was this struct should exactly match the descriptor struct (otherwise, why have the reserved fields?). Should we have a separate copy for the compiler implementation detail version?

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