glsl/glcpp: Add an explanatory comment for "loc != NULL" check

glsl/glcpp: Add an explanatory comment for "loc != NULL" check

Just reading the code, it looked like a bug that _define_object_macro had this
check, but _define_function_macro did not. Upon further reading, that's
because the check is to allow for our builtins to be defined, (and there are
no builtin function-like macros).

Add my new understanding as a comment to help the next reader.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>