Normalize parameters names like _$N to _

TruffleRuby adds a "$N" suffix to parameter names to make them unique,
but that is only possible if the parameter name is `_`. This internal implementation detail then leaks to the user when the user calls `parameters` on a method or proc that has multiple parameters
defined with the name `_`.

This commit matches parameter names that start with `_$` and normalizes
them to be `_` as they are being assembled into the `parameters` array.

Author: paracycle

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

@@ -47,6 +47,11 @@ private static RubyArray toArray(RubyLanguage language, RubyContext context, Arg
         if (argType.anonymous || name == null) {
             store = new Object[]{ language.getSymbol(argType.symbolicName) };
         } else {
+            // make sure to normalize parameter names to "_" if they start with "_$"
+            if (name.startsWith("_$")) {
+                name = "_";
+            }
+
             store = new Object[]{ language.getSymbol(argType.symbolicName), language.getSymbol(name) };
         }