Add OpenCL/SPIR kernel_arg_base_type metadata node

As defined in the SPIR 1.2 specification, this node behaves similarly to
kernel_arg_type but will print the underlying type name, e.g., without
typedefs.

Example:
  typedef unsigned int myunsignedint;
would report:
  'myunsignedint' in the kernel_arg_type node
  'uint' in the kernel_arg_base_type node