glsl/glcpp: Stop using a lexer start condition (<SKIP>) for token skipping.

glsl/glcpp: Stop using a lexer start condition (<SKIP>) for token skipping.

Here, "skipping" refers to the lexer not emitting any tokens for portions of
the file within an #if condition (or similar) that evaluates to false.

Previously, the lexer had a special <SKIP> start condition used to control
this skipping. This start condition was not handled like a normal start
condition. Instead, there was a particularly ugly block of code set to be
included at the top of the generated lexing loop that would change from
<INITIAL> to <SKIP> or from <SKIP> to <INITIAL> depending on various pieces of
parser state, (such as parser->skip_state and parser->lexing_directive).

Not only was that an ugly approach, but the <SKIP> start condition was
complicating several glcpp bug fixes I attempted recently that want to use
start conditions for other purposes, (such as a new <HASH> start condition).

The recently added RETURN_TOKEN macro gives us a convenient way to implement
skipping without using a lexer start condition. Now, at the top of the
generated lexer, we examine all the necessary parser state and set a new
parser->skipping bit. Then, in RETURN_TOKEN, we examine parser->skipping to
determine whether to actually emit the token or not.

Besides this, there are only a couple of other places where we need to examine
the skipping bit (other than when returning a token):

	* To avoid emitting an error for #error if skipped.

	* To avoid entering the <DEFINE> start condition for a #define that is
          skipped.

With all of this in place in the present commit, there are hopefully no
behavioral changes with this patch, ("make check" still passes all of the
glcpp tests at least).

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