stacked-contexts: Add peek() method

gcc/rust/ChangeLog:

	* util/rust-stacked-contexts.h: Add new method to see what context we are currently in.

Author: CohenArthur

gcc/rust/util/rust-stacked-contexts.h

@@ -71,6 +71,13 @@ template <typename T> class StackedContexts
     return last;
   }
 
+  const T &peek ()
+  {
+    rust_assert (!stack.empty ());
+
+    return stack.back ();
+  }
+
   /**
    * Are we currently inside of a special context?
    */