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>

src/glsl/ast.h

       subexpressions[0] = NULL;
       subexpressions[1] = NULL;
       subexpressions[2] = NULL;
-      primary_expression.identifier = (char *) identifier;
+      primary_expression.identifier = identifier;
       this->non_lvalue_description = NULL;
    }
 
    ast_expression *subexpressions[3];
 
    union {
-      char *identifier;
+      const char *identifier;
       int int_constant;
       float float_constant;
       unsigned uint_constant;
 
 class ast_declaration : public ast_node {
 public:
-   ast_declaration(char *identifier, int is_array, ast_expression *array_size,
+   ast_declaration(const char *identifier, int is_array, ast_expression *array_size,
 		   ast_expression *initializer);
    virtual void print(void) const;
 
-   char *identifier;
+   const char *identifier;
    
    int is_array;
    ast_expression *array_size;
 
 class ast_struct_specifier : public ast_node {
 public:
-   ast_struct_specifier(char *identifier, ast_node *declarator_list);
+   ast_struct_specifier(const char *identifier, ast_node *declarator_list);
    virtual void print(void) const;
 
    virtual ir_rvalue *hir(exec_list *instructions,
 			  struct _mesa_glsl_parse_state *state);
 
-   char *name;
+   const char *name;
    exec_list declarations;
 };
 
 			  struct _mesa_glsl_parse_state *state);
 
    ast_fully_specified_type *type;
-   char *identifier;
+   const char *identifier;
    int is_array;
    ast_expression *array_size;
 
 			  struct _mesa_glsl_parse_state *state);
 
    ast_fully_specified_type *return_type;
-   char *identifier;
+   const char *identifier;
 
    exec_list parameters;
 

src/glsl/glsl_parser.yy

 %union {
    int n;
    float real;
-   char *identifier;
+   const char *identifier;
 
    struct ast_type_qualifier type_qualifier;
 

src/glsl/glsl_parser_extras.cpp

 }
 
 
-ast_declaration::ast_declaration(char *identifier, int is_array,
+ast_declaration::ast_declaration(const char *identifier, int is_array,
 				 ast_expression *array_size,
 				 ast_expression *initializer)
 {
 }
 
 
-ast_struct_specifier::ast_struct_specifier(char *identifier,
+ast_struct_specifier::ast_struct_specifier(const char *identifier,
 					   ast_node *declarator_list)
 {
    if (identifier == NULL) {