glsl: Use (const char *) in AST nodes rather than plain (char *).

glsl: Use (const char *) in AST nodes rather than plain (char *).

Nothing actually relied on them being mutable, and there was at least
one cast which discarded const qualifiers.  The next patch would have
introduced many more.

Casting away const qualifiers should be avoided if at all possible.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>