[Foundation] Override mutable copying for CharacterSet

Some mutation cases will cause the underlying copy on write cases to crash with a  _SwiftNSCharacterSet doesn't respond to -mutableCopyWithZone: failure.
Fixes Bugs:
https://bugs.swift.org/browse/SR-1782
<rdar://problem/26608216>

Author: phausler

stdlib/public/SDK/Foundation/Boxing.swift

@@ -116,7 +116,7 @@ extension _SwiftNativeFoundationType {
     }
 
     func mutableCopy(with zone : NSZone) -> AnyObject {
-        return _mapUnmanaged { ($0 as AnyObject).mutableCopy(with: zone) }
+        return _mapUnmanaged { $0.mutableCopy() }
     }
 
     var hashValue: Int {

stdlib/public/SDK/Foundation/CharacterSet.swift

@@ -49,6 +49,16 @@ internal final class _SwiftNSCharacterSet : _SwiftNativeNSCharacterSet, _SwiftNa
     deinit {
         releaseWrappedObject()
     }
+
+    @objc(copyWithZone:)
+    func copy(with zone: NSZone? = nil) -> AnyObject {
+        return _mapUnmanaged { $0.copy(with: zone) }
+    }
+
+    @objc(mutableCopyWithZone:)
+    func mutableCopy(with zone: NSZone? = nil) -> AnyObject {
+        return _mapUnmanaged { $0.mutableCopy(with: zone) }
+    }
 }
 
 /**

test/1_stdlib/TestCharacterSet.swift

@@ -74,7 +74,17 @@ class TestCharacterSet : TestCharacterSetSuper {
         expectTrue(secondCharacterSet.contains(capitalC), "Character set must not contain our letter")
         expectTrue(thirdCharacterSet.contains(capitalC), "Character set must contain our letter")
     }
+
+    func testMutability_mutableCopyCrash() {
+        let cs = CharacterSet(charactersIn: "ABC")
+        (cs as NSCharacterSet).mutableCopy() // this should not crash
+    }
 
+    func testMutability_SR_1782() {
+        var nonAlphanumeric = CharacterSet.alphanumerics.inverted
+        nonAlphanumeric.remove(charactersIn: " ") // this should not crash
+    }
+
     func testRanges() {
         // Simple range check
         let asciiUppercase = CharacterSet(charactersIn: UnicodeScalar(0x41)...UnicodeScalar(0x5A))
@@ -140,6 +150,8 @@ class TestCharacterSet : TestCharacterSetSuper {
 var CharacterSetTests = TestSuite("TestCharacterSet")
 CharacterSetTests.test("testBasicConstruction") { TestCharacterSet().testBasicConstruction() }
 CharacterSetTests.test("testMutability_copyOnWrite") { TestCharacterSet().testMutability_copyOnWrite() }
+CharacterSetTests.test("testMutability_mutableCopyCrash") { TestCharacterSet().testMutability_mutableCopyCrash() }
+CharacterSetTests.test("testMutability_SR_1782") { TestCharacterSet().testMutability_SR_1782() }
 CharacterSetTests.test("testRanges") { TestCharacterSet().testRanges() }
 CharacterSetTests.test("testInsertAndRemove") { TestCharacterSet().testInsertAndRemove() }
 CharacterSetTests.test("testBasics") { TestCharacterSet().testBasics() }