[cxx-interop] Add test for mutable std.optional.pointee

See #67648

Author: egorzhdan

test/Interop/Cxx/stdlib/use-std-optional.swift

@@ -12,6 +12,10 @@ StdOptionalTestSuite.test("pointee") {
   let nonNilOpt = getNonNilOptional()
   let pointee = nonNilOpt.pointee
   expectEqual(123, pointee)
+
+  var modifiedOpt = getNilOptional()
+  modifiedOpt.pointee = 777
+  expectEqual(777, modifiedOpt.pointee)
 }
 
 StdOptionalTestSuite.test("std::optional => Swift.Optional") {