* 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
NSString has a stricter concept of equality than Swift’s String, so it is possible to construct NSDictionary/NSSet instances that contain duplicate keys according to Swift’s definition of string equality. This change improves how we handle these cases by coalescing the duplicate keys (keeping a single value).
rdar://problem/35995647
* replace unused closure parameters with '_' in stdlib source
* fold some _ closure arguments into line above
* fold more _ closure arguments into line above