swift234

AST: Avoid instantiating all members when implicit destructors are inserted into classes.

We were accidentally forcing all members of a class to be instantiated in two places:

- by trying to look up an existing destructor decl in the class, and
- by adding the implicit destructor to the class, because addMember needlessly called loadAllMembers.

Fix the former problem by adding a 'has destructor' bit to ClassDecl so we can track whether the implicit destructor needs to be added without querying its members. Fix the latter by making IterableDeclContext::addMember not call loadAllMembers, and making loadAllMembers not barf when it sees existing members in the context.

Together with Jordan and JoeP's changes, this makes many interpreter tests now compile 3-20x faster.

Swift SVN r17562