utils/types: toggle_set: add "~~" semantics

Add support for "~~" special value that indicates that, when merging
into another set, all values from that set should in fact be dropped.
Apart from the unique merge semantics, "~~" just like any other "~"
value.

wa/framework/configuration/core.py

@@ -957,6 +957,8 @@ class JobGenerator(object):
 
     def disable_augmentations(self, augmentations):
         for entry in augmentations:
+            if entry == '~~':
+                continue
             if entry.startswith('~'):
                 entry = entry[1:]
             try: