glsl/glcpp: Fix handling of commas that result from macro expansion

glsl/glcpp: Fix handling of commas that result from macro expansion

Here is some additional stress testing of nested macros where the expansion
of macros involves commas, (and whether those commas are interpreted as
argument separators or not in subsequent function-like macro calls).

Credit to the GCC documentation that directed my attention toward this issue:

	https://gcc.gnu.org/onlinedocs/gcc-3.2/cpp/Argument-Prescan.html

Fixing the bug required only removing code from glcpp. When first testing the
details of expansions involving commas, I had come to the mistaken conclusion
that an expanded comma should never be treated as an argument separator, (so
had introduced the rather ugly COMMA_FINAL token to represent this).

In fact, an expanded comma should be treated as a separator, (as tested here),
and this treatment can be avoided by judicious use of parentheses (as also
tested here).

With this simple removal of the COMMA_FINAL token, the behavior of glcpp
matches that of gcc's preprocessor for all of these hairy cases.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>