Fix StringTransform usage of forced casts.

In Linux, a (constant?) CFString cannot be force casted to String
directly. The code generated for those values with optimizations enabled
was simply a crash. The changes uses `_swiftObject` to correctly
transform to a `Swift.String`.

There some other cases in Foundation where other enums are initialized
from CF values, but in those cases the literals of the CF values are
used, which avoids this problem.

Author: drodriguez

Foundation/NSString.swift

@@ -135,34 +135,34 @@ extension NSString {
 
 // Transliteration
 extension NSString.StringTransform {
-  public static let toLatin: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformToLatin! as! String)
-  public static let latinToArabic: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinArabic! as! String)
-  public static let latinToCyrillic: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinCyrillic! as! String)
-  public static let latinToGreen: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinGreek! as! String)
-  public static let latinToHangul: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinHangul! as! String)
-  public static let latinToHebrew: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinHebrew! as! String)
-  public static let latinToHiragana: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinHiragana! as! String)
-  public static let latinToKatakana: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinKatakana! as! String)
-  public static let latinToThai: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinThai! as! String)
-  public static let hiraganaToKatakana: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformHiraganaKatakana! as! String)
-  public static let mandarinToLatin: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformMandarinLatin! as! String)
+  public static let toLatin: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformToLatin!._swiftObject)
+  public static let latinToArabic: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinArabic!._swiftObject)
+  public static let latinToCyrillic: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinCyrillic!._swiftObject)
+  public static let latinToGreen: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinGreek!._swiftObject)
+  public static let latinToHangul: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinHangul!._swiftObject)
+  public static let latinToHebrew: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinHebrew!._swiftObject)
+  public static let latinToHiragana: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinHiragana!._swiftObject)
+  public static let latinToKatakana: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinKatakana!._swiftObject)
+  public static let latinToThai: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformLatinThai!._swiftObject)
+  public static let hiraganaToKatakana: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformHiraganaKatakana!._swiftObject)
+  public static let mandarinToLatin: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformMandarinLatin!._swiftObject)
 }
 
 // Diacritic and Combining Mark Removal
 extension NSString.StringTransform {
-  public static let stripDiacritics: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformStripDiacritics! as! String)
-  public static let stripCombiningMarks: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformStripCombiningMarks! as! String)
+  public static let stripDiacritics: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformStripDiacritics!._swiftObject)
+  public static let stripCombiningMarks: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformStripCombiningMarks!._swiftObject)
 }
 
 // Halfwidth and Fullwidth Form Conversion
 extension NSString.StringTransform {
-  public static let fullwidthToHalfwidth: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformFullwidthHalfwidth! as! String)
+  public static let fullwidthToHalfwidth: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformFullwidthHalfwidth!._swiftObject)
 }
 
 // Character Representation
 extension NSString.StringTransform {
-  public static let toUnicodeName: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformToUnicodeName! as! String)
-  public static let toXMLHex: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformToXMLHex! as! String)
+  public static let toUnicodeName: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformToUnicodeName!._swiftObject)
+  public static let toXMLHex: NSString.StringTransform = NSString.StringTransform(rawValue: kCFStringTransformToXMLHex!._swiftObject)
 }
 
 internal func _createRegexForPattern(_ pattern: String, _ options: NSRegularExpression.Options) -> NSRegularExpression? {