Update comments for expressions split due to slow type checking.

Add the SR number to the comment. I've also updated the SR to point back
to the PR for the work-around so that we know to revert the change when
the issue is resolved.

Author: rudkx

Foundation/NSCFString.swift

@@ -59,7 +59,8 @@ internal class _NSCFString : NSMutableString {
 
 internal final class _NSCFConstantString : _NSCFString {
     internal var _ptr : UnsafePointer<UInt8> {
-        // FIXME: Split expression as a work-around for slow type checking.
+        // FIXME: Split expression as a work-around for slow type
+        //        checking (tracked by SR-5322).
         let offTemp1 = MemoryLayout<OpaquePointer>.size + MemoryLayout<Int32>.size
         let offTemp2 = MemoryLayout<Int32>.size + MemoryLayout<_CFInfo>.size
         let offset = offTemp1 + offTemp2
@@ -68,7 +69,8 @@ internal final class _NSCFConstantString : _NSCFString {
     }
 
     private var _lenOffset : Int {
-        // FIXME: Split expression as a work-around for slow type checking.
+        // FIXME: Split expression as a work-around for slow type
+        //        checking (tracked by SR-5322).
         let offTemp1 = MemoryLayout<OpaquePointer>.size + MemoryLayout<Int32>.size
         let offTemp2 = MemoryLayout<Int32>.size + MemoryLayout<_CFInfo>.size
         return offTemp1 + offTemp2 + MemoryLayout<UnsafePointer<UInt8>>.size