[clang] [clang][CodeGen][SPIRV] Translate `amdgpu_flat_work_group_size` into `reqd_work_group_size`. (PR #116820)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I see. Basically we redefined the semantic of reqd_work_group_size for HIP-generated SPIRV. Do we have a way to differentiate OpenCL-generated and HIP-generated SPIRV? They will be translated differently about reqd_work_group_size

There shouldn't be one. Representation needs to be consistent and language / producer / consumer independent 

https://github.com/llvm/llvm-project/pull/116820