mesa: Add string_to_uint_map::clear method to clear the map

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Tom Stellard <thomas.stellard@amd.com>

src/mesa/program/hash_table.h

@@ -213,6 +213,14 @@ public:
       hash_table_dtor(this->ht);
    }
 
+   /**
+    * Remove all mappings from this map.
+    */
+   void clear()
+   {
+      hash_table_clear(this->ht);
+   }
+
    /**
     * Get the value associated with a particular key
     *