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.
...being careful to only do it once per initializer. Additionally,
/don't/ offer the suggestion if there was already a conditional
assignment to 'self', because that would wipe it out and the user
should think harder.
(and when the struct in question is non-fixed-layout, which was
already implemented)
This ensures that these initializers are never fieldwise in Swift 5
mode, which makes it safe for library authors to add new fields.
...unless the struct contains a field that cannot be zero-initialized,
such as a non-nullable pointer.
This suggestion is only made for C structs because 'init()' may not be
the right choice for other structs.
...as detected by initializing an individual field without having
initialized the whole object (via `self = value`).
This only applies in pre-Swift-5 mode because the next commit will
treat all cross-module struct initializers as delegating in Swift 5.
