Update Cxx library for strict memory safety

Author: DougGregor

stdlib/public/Cxx/CxxSpan.swift

@@ -95,7 +95,7 @@ extension Span {
     _unsafeCxxSpan span: borrowing T,
   ) {
     let buffer = unsafe UnsafeBufferPointer(start: span.__dataUnsafe(), count: Int(span.size()))
-    let newSpan = Span(_unsafeElements: buffer)
+    let newSpan = unsafe Span(_unsafeElements: buffer)
     // 'self' is limited to the caller's scope of the variable passed to the 'span' argument.
     self = unsafe _overrideLifetime(newSpan, borrowing: span)
   }