Fix unnecessary mutability of buffer elements.

Author: atrick

stdlib/public/core/ContiguousArrayBuffer.swift

@@ -115,7 +115,7 @@ final class _ContiguousArrayStorage<Element> : _ContiguousArrayStorage1 {
   ) rethrows {
     if _isBridgedVerbatimToObjectiveC(Element.self) {
       let count = __manager.header.count
-      let elements = UnsafeMutableRawPointer(__manager._elementPointer)
+      let elements = UnsafeRawPointer(__manager._elementPointer)
         .assumingMemoryBound(to: AnyObject.self)
       defer { _fixLifetime(__manager) }
       try body(UnsafeBufferPointer(start: elements, count: count))