[mlir][gpu] Modifies `gpu.launch_func` to allow lowering it after gpu-to-llvm.

**For an explanation of these patches see D154153.**

Commit message:
In order to lower `gpu.launch_func` after running `gpu-to-llvm` it must be
able to handle lowered types -eg. index -> i64. This patch also allows the op
to refer to GPU binaries and not only GPU modules.

Depends on D154132.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D154137