test: repair Autodiff/validation-test/optional-property on Win32

The test relies on the module name being `null`.  This is implied by the
`-o /dev/null`.  However, that is not guaranteed.  Explicitly use the
desired module name.

Author: compnerd

test/AutoDiff/validation-test/optional-property.swift

@@ -1,5 +1,5 @@
 // RUN: %target-run-simple-swift
-// RUN: %target-swift-emit-sil -Xllvm -debug-only=differentiation -o /dev/null 2>&1 %s | %FileCheck %s
+// RUN: %target-swift-emit-sil -Xllvm -debug-only=differentiation -module-name null -o /dev/null 2>&1 %s | %FileCheck %s
 // REQUIRES: executable_test
 // REQUIRES: asserts