swift234

[stdlib] Avoid Suicide Sequences

In a naive implementation of a Sequence adapter, adapting a
non-self-destructive (multi-pass) Sequence having a reference-semantics
Generator produces a self-destructive adapted view.  This is technically
correct because Sequences are allowed to be self-destructive, and
theoretically every multi-pass Sequence would be a Collection.  However
Sequences are much easier to build than Collections, so users are likely
to make them, and it would be extremely surprising if adapting a
self-preserving Sequence yielded a self-destructive one.

Swift SVN r18350