GPU Dialect: introduce gpu.return

This terminator operation should appear at the end of the blocks in the body
region of `gpu.launch` when the control flow needs to be returned from the
kernel.  Using `std.return` in this place is ambiguous: it may exit the body
region or the enclosing function.  Furthermore, this allows the GPU dialect to
impose the absence of return values as required by the underlying kernel
execution models.

Update outlining transformation from `gpu.launch` to `gpu.launch_func` so that
it replaces `gpu.return` with `std.return`.

PiperOrigin-RevId: 252985992