Improve auto push_constants upgrading.

[mbechard]

Ensure all copies of the global uniform block symbol get updated with the correct layoutPacking when it's auto upgraded to a push_constant. In some cases there are multiple copies, and only one was getting changed previously.

[arcady-lunarg]

Would it make sense to add a unit test for this? In what case was the problem being triggered?

[mbechard]

I've only see it occur in one huge shader. It's from an end-user so I don't think they'd want it posted to the repository. I'm not sure under what circumstances the tree ends up with duplicated symbols, but this fix follows other similar workflows at least.
I also don't think the testing infrastructure is setup to test this kind of stuff anyways. We'd need a way to apply different rules to different test cases I think.

[arcady-lunarg]

Looks fine.