[OpenCL] Reject block arguments

OpenCL 2.0 does not allow block arguments, primarily because it is
difficult to support function pointers on the various architectures
that OpenCL targets.  Clang was still accepting them.

Rename and reuse the `err_opencl_half_param` diagnostic.

Fixes PR46324.

Differential Revision: https://reviews.llvm.org/D82313