Always emit kernel arg info for SPIR.

http://llvm.org/bugs/show_bug.cgi?id=21555

Currently, kernel argument metadata is omitted unless the
"-cl-kernel-arg-info" option is specified. But the SPIR 1.2 spec
requires that all metadata except kernel_arg_name should always be
emitted, and kernel_arg_name is only emitted when
"-cl-kernel-arg-info" is specified.

Patch ported by Ryan Burn from the Khronos SPIR generator.
https://github.com/KhronosGroup/SPIR