Fix lengthOfBytes(using:) semantics (#83334)

Fixes rdar://156675395

Author: Catfish-Man

stdlib/public/core/StringStorageBridge.swift

@@ -106,7 +106,7 @@ extension _AbstractStringStorage {
     case _cocoaUTF8Encoding:
       return UInt(count)
     case _cocoaUTF16Encoding:
-      return UInt(UTF16Length)
+      return UInt(UTF16Length) * 2
     case _cocoaMacRomanEncoding:
       if unsafe isASCII || _allASCII(UnsafeBufferPointer(start: start, count: count)) {
         return UInt(count)

test/stdlib/StringBridge.swift

@@ -174,5 +174,25 @@ StringBridgeTests.test("Character from NSString") {
   expectNil((ns3 as String).utf8.withContiguousStorageIfAvailable(returnOne))
 }
 
+StringBridgeTests.test("lengthOfBytes(using:)") {
+  let ascii = "The quick brown fox jumps over the lazy dog"
+  let utf8 = "The quick brown fox jümps over the lazy dog"
+  let asciiAsASCIILen = ascii.lengthOfBytes(using: .ascii)
+  let asciiAsUTF8Len = ascii.lengthOfBytes(using: .utf8)
+  let asciiAsUTF16Len = ascii.lengthOfBytes(using: .utf16)
+  let asciiAsMacRomanLen = ascii.lengthOfBytes(using: .macOSRoman)
+  let utf8AsASCIILen = utf8.lengthOfBytes(using: .ascii)
+  let utf8AsUTF8Len = utf8.lengthOfBytes(using: .utf8)
+  let utf8AsUTF16Len = utf8.lengthOfBytes(using: .utf16)
+  let utf8AsMacRomanLen = utf8.lengthOfBytes(using: .macOSRoman)
+  expectEqual(asciiAsASCIILen, 43)
+  expectEqual(asciiAsUTF8Len, 43)
+  expectEqual(asciiAsUTF16Len, 86)
+  expectEqual(asciiAsMacRomanLen, 43)
+  expectEqual(utf8AsASCIILen, 0)
+  expectEqual(utf8AsUTF8Len, 44)
+  expectEqual(utf8AsUTF16Len, 86)
+  expectEqual(utf8AsMacRomanLen, 43)
+}
 
 runAllTests()