Right now the stdlib/overlays can compile against -Onone tests with or without
-enable-ownership-stripping-after-serialization. This will help me to prevent
other work going on from breaking these properties.
yields in generalized accessors: _read and _modify, which are
yield-once corountines. This pass is based on the existing SIL verifier
checks but diagnoses only those errors that can be introduced by programmers
when using yields.
<rdar://43578476>
