Support kernels returning Union{}

[maleadt]

Now that we support some exceptions, we should treat these kernels as valid and just have them throw at run time. This should make it much easier to debug issues like that -- assuming we can report something useful for eg. MethodErrors.

[maleadt]

Interesting example: method ambiguities. Result in a returned bottom, but it should be able to spot them at IR validation time.

[maleadt]

This is allowed now:

julia> @device_code_warntype @cuda (()->throw(1))()
PTX CompilerJob of MethodInstance for (::var"#11#12")() for sm_86

MethodInstance for (::var"#11#12")()
  from (::var"#11#12")() @ Main REPL[33]:1
Arguments
  #self#::Core.Const(var"#11#12"())
Body::Union{}
1 ─     Main.throw(1)
└──     Core.Const(:(return %1))


julia> ERROR: a exception was thrown during kernel execution on thread (1, 1, 1) in block (1, 1, 1).
Stacktrace not available, run Julia on debug level 2 for more details (by passing -g2 to the executable).