Add query to determine whether a name was declared at this scope

This will be used to prevent a variable and a function with the same
name from being declared.  As a side effect, the calls to
add_{type,name,function} should never fail.

glsl_symbol_table.h

@@ -65,6 +65,14 @@ public:
       _mesa_symbol_table_pop_scope(table);
    }
 
+   /**
+    * Determine whether a name was declared at the current scope
+    */
+   bool name_declared_this_scope(const char *name)
+   {
+      return _mesa_symbol_table_symbol_scope(table, -1, name) == 0;
+   }
+
    /**
     * \name Methods to add symbols to the table
     *