[llvm] [AMDGPU] Compiler should synthesize private buffer resource descriptor from flat_scratch_init (PR #79586)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

We do need a follow up change to prune this from the calling convention lowering. It's going to cause a huge amount of test churn, so it should be done separately 

https://github.com/llvm/llvm-project/pull/79586