* Make _sanityCheck internal
* Make _debugPrecondition internal
* Make Optional._unsafelyUnwrappedUnchecked internal.
* Make _precondition internal
* Switch Foundation _sanityChecks to assertions
* Update file check tests
* Remove one more _debugPrecondition
* Update Optimization-with-check tests
...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.
