[stdlib] add _withUnprotectedUnsafeMutableBytes()

Author: glessard

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -1230,6 +1230,24 @@ func __abi_se0413_withUnsafeMutableBytes<T, Result>(
   }
 }
 
+/// Invokes the given closure with a buffer pointer covering the raw bytes of
+/// the given argument.
+///
+/// This function is similar to `withUnsafeMutableBytes`, except that it
+/// doesn't trigger stack protection for the pointer.
+@_alwaysEmitIntoClient
+public func _withUnprotectedUnsafeMutableBytes<T, E: Error, Result>(
+  of value: inout T,
+  _ body: (UnsafeMutableRawBufferPointer) throws(E) -> Result
+) throws(E) -> Result {
+#if $BuiltinUnprotectedAddressOf
+  let pointer = UnsafeMutableRawPointer(Builtin.unprotectedAddressOf(&value))
+#else
+  let pointer = UnsafeMutableRawPointer(Builtin.addressof(&value))
+#endif
+  return try body(.init(start: pointer, count: MemoryLayout<T>.size))
+}
+
 /// Invokes the given closure with a buffer pointer covering the raw bytes of
 /// the given argument.
 ///