[stdlib] Factor a couple more universal inits into _Pointer

[airspeedswift]

All pointers can be created from an UnsafeMutablePointer, so those can move to _Pointer

[airspeedswift]

@swift-ci please test

[airspeedswift]

@atrick one side effect of this is that it's stomped on some of the notes we produce to nudge people towards using .assumingMemoryBound(to:). Do you see those as just supporting the transition back in Swift 3? Or more a forever-important note?

[airspeedswift]

@swift-ci please test source compatibility

[lilyball]

I think this PR needs to be reverted (except I guess for the seemingly-unrelated @inlinable tweaks in BridgeObjectiveC.swift). This introduces initializers on UnsafePointer and UnsafeMutablePointer that don't belong (see SR-9800 for details).

Also it looks like you forgot to remove these initializers from UnsafeMutableRawPointer, that still redeclares these initializers.

[atrick]

Wow, huge thanks to @lilyball for pointing out a problem that I incredibly overlooked before it became a really big problem.