Tweaks to compare-and-exchange functions

Move choice between weak and strong CAS out of the C parameter list in CAtomics

• In the CAtomics module, all types have direct CompareAndExchangeWeak and CompareAndExchangeStrong functions rather than a single function with a parameter selecting between strong and weak.

• The old version of CompareAndExchange with a CASType parameter still exists, but forwards to either of the other two, making it objectively worse.

• SwiftAtomics internally adapts to the above, but is otherwise unchanged.