Implement String#-@ and untag passing specs.

Author: aardvark179

spec/tags/core/string/uminus_tags.txt

@@ -1,3 +1 @@
-fails:String#-@ returns the same object for equal unfrozen strings
-fails:String#-@ returns the same object when it's called on the same String literal
 fails:String#-@ deduplicates frozen strings

src/main/ruby/core/string.rb

@@ -1590,7 +1590,8 @@ def +@
   end
 
   def -@
-    frozen? ? self : dup.freeze
+    str = frozen? ? self : dup.freeze
+    Truffle::StringOperations.intern_string(str)
   end
 
   def encoding

src/main/ruby/core/truffle/string_operations.rb

@@ -11,6 +11,12 @@
 module Truffle
   module StringOperations
 
+    FROZEN_STRING_HASH = {}
+
+    def self.intern_string(str)
+      FROZEN_STRING_HASH[str] ||= str
+    end
+
     # Similar to MRI's Warning::buffer class
     class SimpleStringIO
       attr_reader :string