[string] Wean off of no-longer-supported String non-API

String's _core disappeared, but we kept around a mock-up of it as a
faux-SPI to avoid breaking Foundation. However, it does not fully
model String, and the illusion is broken from small string support.

Author: milseman

Foundation/NSString.swift

@@ -296,27 +296,14 @@ open class NSString : NSObject, NSCopying, NSMutableCopying, NSSecureCoding, NSC
     }
 
     internal func _fastCStringContents(_ nullTerminated: Bool) -> UnsafePointer<Int8>? {
-        if type(of: self) == NSString.self || type(of: self) == NSMutableString.self {
-            if _storage._core.isASCII {
-                return unsafeBitCast(_storage._core.startASCII, to: UnsafePointer<Int8>.self)
-            }
-        }
         return nil
     }
 
     internal var _fastContents: UnsafePointer<UniChar>? {
-        if type(of: self) == NSString.self || type(of: self) == NSMutableString.self {
-            if !_storage._core.isASCII {
-                return UnsafePointer<UniChar>(_storage._core.startUTF16)
-            }
-        }
         return nil
     }
 
     internal var _encodingCantBeStoredInEightBitCFString: Bool {
-        if type(of: self) == NSString.self || type(of: self) == NSMutableString.self {
-            return !_storage._core.isASCII
-        }
         return false
     }
 
@@ -857,17 +844,6 @@ extension NSString {
 
     public func getCString(_ buffer: UnsafeMutablePointer<Int8>, maxLength maxBufferCount: Int, encoding: UInt) -> Bool {
         var used = 0
-        if type(of: self) == NSString.self || type(of: self) == NSMutableString.self {
-            if _storage._core.isASCII {
-                used = min(self.length, maxBufferCount - 1)
-                _storage._core.startASCII.withMemoryRebound(to: Int8.self,
-                                                            capacity: used) {
-                    buffer.moveAssign(from: $0, count: used)
-                }
-                buffer.advanced(by: used).initialize(to: 0)
-                return true
-            }
-        }
         if getBytes(UnsafeMutableRawPointer(buffer), maxLength: maxBufferCount, usedLength: &used, encoding: encoding, options: [], range: NSRange(location: 0, length: self.length), remaining: nil) {
             buffer.advanced(by: used).initialize(to: 0)
             return true