[Foundation] Address exclusivity violation in Data

Address an exclusivity violation in Data's Iterator.next() by changing two private
'let' stored properties to be 'var'.

Making the properties 'var' changes code generation of next() so that the stored
properties are read independently of the other contents of the struct. This
prevents an exclusivity violation when reading '_endIdx' and '_data' while
simultaneously mutating '_buffer' with the call to withUnsafeMutablePointer().

The 'let' pattern is an idiom we would eventually like to support (see SR-7396),
but for now we need to remove the exclusivity violation.

Author: devincoughlin

stdlib/public/SDK/Foundation/Data.swift

@@ -1762,14 +1762,23 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
     }
 
     public struct Iterator : IteratorProtocol {
-        private let _data: Data
+        // Both _data and _endIdx should be 'let' rather than 'var'.
+        // They are 'var' so that the stored properties can be read
+        // independently of the other contents of the struct. This prevents
+        // an exclusivity violation when reading '_endIdx' and '_data'
+        // while simultaneously mutating '_buffer' with the call to
+        // withUnsafeMutablePointer(). Once we support accessing struct
+        // let properties independently we should make these variables
+        // 'let' again.
+
+        private var _data: Data
         private var _buffer: (
         UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
         UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
         UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
         UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8)
         private var _idx: Data.Index
-        private let _endIdx: Data.Index
+        private var _endIdx: Data.Index
 
         fileprivate init(_ data: Data) {
             _data = data