gccrs: Add const getter for tokentrees

We often need to retrieve the underlying tokentree without modifying it,
this getter will help achieve this.

gcc/rust/ChangeLog:

	* ast/rust-ast.h: Add const getter.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

gcc/rust/ast/rust-ast.h

@@ -901,6 +901,11 @@ public:
     return token_trees;
   }
 
+  const std::vector<std::unique_ptr<TokenTree>> &get_token_trees () const
+  {
+    return token_trees;
+  }
+
   DelimType get_delim_type () const { return delim_type; }
 };