[mlir][GPU] Use StructAttr to drive lowering from loop.parallel to

gpu.launch

Current implementation of lowering from loop.parallel to gpu.launch
uses a DictionaryAttr to specify the mapping. Moving this attribute to
be auto-generated from specification as a StructAttr. This simplifies
a lot the logic of looking up and creating this attribute.

Differential Revision: https://reviews.llvm.org/D76165