swift234

Runtime: Add ExistentialTypeMetadata helpers for taking values.

Add a 'mayTakeValue' function, which returns true if we're allowed to take the value from an existential container (assuming we own the existential container value). This is true for class and opaque existential containers, but not for boxed existentials, where multiple owners may share references to the boxed value. Also add a 'deinitExistentialContainer' function to do any cleanup that must happen after the value has been consumed out of an existential container. Fix up an off-by-one indirection level in the SwiftError implementations of the existing existential container methods.

Swift SVN r26561