...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.
When we give NSNumber a custom AnyHashable representation, we want to
give it as large a box as possible. When we want to compare it against
other AnyHashable boxes such as Int or UInt, it's always possible to
upcast the Int/UInt to a larger integer size like Int64 or UInt64 for
the comparison. By eliminating the smaller boxes we create, we can
maintain the existing behavior that _SwiftTypePreservingNSNumber gave
us.
