glsl/glcpp: Add (non)-support for ++ and -- operators

glsl/glcpp: Add (non)-support for ++ and -- operators

These operators aren't defined for preprocessor expressions, so we never
implemented them. This led them to be misinterpreted as strings of unary
'+' or '-' operators.

In fact, what is actually desired is to generate an error if these operators
appear in any preprocessor condition.

So this commit looks like it is strictly adding support for these
operators. And it is supporting them as far as passing them through to the
subsequent compiler, (which was already happening anyway).

What's less apparent in the commit is that with these tokens now being lexed,
but with no change to the grammar for preprocessor expressions, these
operators will now trigger errors there.

A new "make check" test is added to verify the desired behavior.

This commit fixes the following Khronos GLES3 CTS test:

	invalid_op_1_vertex
	invalid_op_1_fragment
	invalid_op_2_vertex
	invalid_op_2_fragment

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