[amdgpu] Error, instead of miscompile, anonymous kernels using lds

The association between kernel and struct is done by symbol name.
This doesn't work robustly for anonymous kernels as shown by the modified
test case.

An alternative association between function and struct can be constructed
if necessary, probably though metadata, but on the basis that we currently
miscompile anonymous kernels and that they are difficult to construct from
application code and difficult to call from the runtime, this patch makes
it a fatal error for now.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D134741