[mlir][AMDGPU] Set uniform-work-group-size=true by default (#79077)

GPU kernels generated via typical MLIR mechanisms make the assumption
that all workgroups are of uniform size, and so, as in OpenMP, it is
appropriate to set the "uniform-work-group-size"="true" attribute on
these functions by default. This commit makes that choice.

In the event it is needed, this commit adds
`rocdl.uniform_work_group_size` as an attribute to be set on LLVM
functions that can be used to override the default.

In addition, add proper failure messages to translation