[build-script] Make sure that StageArgs are never passed a StageArgs as their args.

No good reason to do this and simplifies the state space.

Author: gottesmm

utils/swift_build_support/swift_build_support/compiler_stage.py

@@ -14,6 +14,7 @@ class StageArgs(object):
     def __init__(self, stage, args):
         self.stage = stage
         self.args = args
+        assert(not isinstance(self.args, StageArgs))
 
     def __getattr__(self, key):
         real_key = '{}{}'.format(key, self.stage.postfix)