[Bug #20695] Do not create needless string object in parser

`set_parser_s_value` does nothing in parser therefore no need to
create string object in parser `set_yylval_node`.

# Object allocation

Run `ruby benchmarks/lobsters/benchmark.rb` with the patch

```diff
diff --git a/benchmarks/lobsters/benchmark.rb b/benchmarks/lobsters/benchmark.rb
index 240c50c..6cdd0ac 100644
--- a/benchmarks/lobsters/benchmark.rb
+++ b/benchmarks/lobsters/benchmark.rb
@@ -7,6 +7,8 @@ Dir.chdir __dir__
 use_gemfile

 require_relative 'config/environment'
+printf "allocated_after_load=%d\n", GC.stat(:total_allocated_objects)
+exit
 require_relative "route_generator"

 # For an in-mem DB, we need to load all data on every boot
```

## Before

```
ruby 3.4.0dev (2024-08-31T18:30:25Z master d6fc8f3d57) [arm64-darwin21]
...
allocated_after_load=2143519
```

## After

```
ruby 3.4.0dev (2024-09-01T00:40:04Z fix_bugs_20695 d1bae52f75) [arm64-darwin21]
...
allocated_after_load=1579662
```

## Ruby 3.3.0 for reference

```
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin21]
...
allocated_after_load=1732702
```

parse.y

@@ -6908,11 +6908,7 @@ static void tokaddmbc(struct parser_params *p, int c, rb_encoding *enc);
 static enum yytokentype parse_string(struct parser_params*,rb_strterm_literal_t*);
 static enum yytokentype here_document(struct parser_params*,rb_strterm_heredoc_t*);
 
-#ifndef RIPPER
-#define set_parser_s_value(x) (void)(x)
-#else
-#define set_parser_s_value(x) (p->s_value = (x))
-#endif
+#define set_parser_s_value(x) (ifdef_ripper(p->s_value = (x), (void)0))
 
 # define set_yylval_node(x) {				\
   YYLTYPE _cur_loc;					\