syntax: Add tokens: Option<TokenStream> to Item

This commit adds a new field to the `Item` AST node in libsyntax to optionally
contain the original token stream that the item itself was parsed from. This is
currently `None` everywhere but is intended for use later with procedural
macros.

src/libsyntax/diagnostics/plugin.rs

@@ -236,6 +236,7 @@ pub fn expand_build_diagnostic_array<'cx>(ecx: &'cx mut ExtCtxt,
             ),
             vis: ast::Visibility::Public,
             span: span,
+            tokens: None,
         })
     ]))
 }