Replace objc version of getSystemVersionPlistProperty with swift version

The comment in the function said:
// This function is implemented in Objective-C because Swift does not support
// failing initializers.
which hasn't been true since swift 1.1.

Author: porglezomp

stdlib/private/StdlibUnittest/CMakeLists.txt

@@ -29,7 +29,6 @@ add_swift_target_library(swiftStdlibUnittest ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES}
   StringConvertible.swift
   TestHelpers.swift
   TypeIndexed.swift
-  GetOSVersion.mm
 
   SWIFT_MODULE_DEPENDS SwiftPrivate SwiftPrivateThreadExtras SwiftPrivateLibcExtras
   SWIFT_MODULE_DEPENDS_IOS Darwin Foundation

stdlib/private/StdlibUnittest/GetOSVersion.mm

@@ -16,6 +16,7 @@ import SwiftPrivateThreadExtras
 import SwiftPrivateLibcExtras
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+import Foundation
 import Darwin
 #elseif os(Linux) || os(FreeBSD) || os(PS4) || os(Android) || os(Cygwin) || os(Haiku)
 import Glibc
@@ -1712,15 +1713,8 @@ public final class TestSuite {
 }
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
-@_silgen_name("getSystemVersionPlistProperty")
-func _getSystemVersionPlistPropertyImpl(
-  _ propertyName: UnsafePointer<CChar>) -> UnsafePointer<CChar>?
-
 func _getSystemVersionPlistProperty(_ propertyName: String) -> String? {
-  let cs = _getSystemVersionPlistPropertyImpl(propertyName)
-  let str = cs.map(String.init(cString:))
-  free(UnsafeMutableRawPointer(mutating: cs))
-  return str
+  return NSDictionary(contentsOfFile: "/System/Library/CoreServices/SystemVersion.plist")?[propertyName] as? String
 }
 #endif