SR-5727: Sync NSStringAPI.swift from overlay

[ianpartridge]

Over time, NSStringAPI.swift has diverged from the version in the overlay.

This PR brings them back into sync, by copying the overlay version into swift-corelibs-foundation, then fixing the resulting breakages so it builds and passes TestFoundation on Linux and in Xcode.

Hopefully this will make it easier to port changes between the two in future.

This is a second attempt after #1167

[ianpartridge]

@swift-ci please test

[ianpartridge]

@phausler @parkera are you happy with this? It fixes bad things like this:

Linux:

$ swift
Welcome to Swift version 4.0-dev (LLVM 2dedb62a0b, Clang b9d76a314c, Swift 61ad3c07ef). Type :help for assistance.
  1> import Foundation 
  2. let str = "abc" 
  3. str[str.startIndex...].replacingOccurrences(of: "a", with: "A")
error: repl.swift:3:4: error: value of type 'String.SubSequence' (aka 'Substring') has no member 'replacingOccurrences'
str[str.startIndex...].replacingOccurrences(of: "a", with: "A")
~~~^~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~

Darwin:

$ swift
Welcome to Apple Swift version 4.0-dev (LLVM 2dedb62a0b, Clang b9d76a314c, Swift 61ad3c07ef). Type :help for assistance.
  1> import Foundation 
  2. let str = "abc" 
  3. str[str.startIndex...].replacingOccurrences(of: "a", with: "A") 
$R0: String = "Abc"
str: String = "abc"

[ianpartridge]

@parkera Thanks for merging swiftlang/swift#11474 - can we do this one too?

[parkera]

@swift-ci test and merge