vtn: add support for atomic flag test/set/clear

This adds support for SpvOpAtomicFlag operations.

This is just a simple implementation that lowers
Clear to Store 0
and
TestAndSet to Cas (0, -1)

There are likely platforms/hw that will want to
lower this all the way through NIR and into their
backend, but this will do for now.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <!12229>
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