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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm requested changes to this pull request.

This is just wrong. These are not equivalent or translatable between each other.

The flat work group size is 1-dimensional only, and a range of permissible values. reqd_work_group_size is an exact match for all 3 dimensions. The backend already can directly consume reqd_work_group_size

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