Refactor normalizing of _$n variable names and introduce a new String constant for "_$" literal

It's a follow up on #3745

Author: andrykonchin

src/main/java/org/truffleruby/language/arguments/ArgumentDescriptorUtils.java

@@ -19,6 +19,7 @@
 import org.truffleruby.parser.ArgumentType;
 
 import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
+import org.truffleruby.parser.parser.ParserSupport;
 
 public class ArgumentDescriptorUtils {
 
@@ -48,7 +49,7 @@ private static RubyArray toArray(RubyLanguage language, RubyContext context, Arg
             store = new Object[]{ language.getSymbol(argType.symbolicName) };
         } else {
             // make sure to normalize parameter names to "_" if they start with "_$"
-            if (name.startsWith("_$")) {
+            if (name.startsWith(ParserSupport.UNDERSCORE_PREFIX)) {
                 name = "_";
             }
 

src/main/java/org/truffleruby/parser/parser/ParserSupport.java

@@ -160,6 +160,8 @@ public class ParserSupport {
     public static final String FORWARD_ARGS_BLOCK_VAR = Layouts.TEMP_PREFIX + "forward_block";
     public static final TruffleString FORWARD_ARGS_BLOCK_VAR_TSTRING = TStringUtils
             .usAsciiString(FORWARD_ARGS_BLOCK_VAR);
+    /** A prefix for duplicated '_' local variables to build unique names */
+    public static final String UNDERSCORE_PREFIX = "_$";
 
     // Parser states:
     protected StaticScope currentScope;
@@ -1587,7 +1589,7 @@ public ArgumentParseNode arg_var(String string) {
         if (name == "_") {
             int count = 0;
             while (current.exists(name) >= 0) {
-                name = ("_$" + count++).intern();
+                name = (UNDERSCORE_PREFIX + count++).intern(); // e.g. _$1, _$2, etc
             }
         }