[ADT] Use std::nullopt instead of llvm::None in OptionalTest.cpp

[kazu]

Summary

This patch updates the test for the streaming operator for
llvm::Optional to use std::nullopt instead of llvm::None.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

llvm/unittests/ADT/OptionalTest.cpp

friend bool operator==(Comparable, Comparable) LLVM_ATTRIBUTE_USED {
     return true;
   }
   static Optional<Comparable> get() { return Comparable(); }
 };
 
 TEST(OptionalTest, UseInUnitTests) {
   // Test that we invoke the streaming operators when pretty-printing values in
   // EXPECT macros.
-  EXPECT_NONFATAL_FAILURE(EXPECT_EQ(llvm::None, ComparableAndStreamable::get()),
-                          "Expected equality of these values:\n"
-                          "  llvm::None\n"
-                          "    Which is: None\n"
-                          "  ComparableAndStreamable::get()\n"
-                          "    Which is: ComparableAndStreamable");
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_EQ(std::nullopt, ComparableAndStreamable::get()),
+      "  std::nullopt\n"
 
   // Test that it is still possible to compare objects which do not have a
   // custom streaming operator.
-  EXPECT_NONFATAL_FAILURE(EXPECT_EQ(llvm::None, Comparable::get()), "object");
+  EXPECT_NONFATAL_FAILURE(EXPECT_EQ(std::nullopt, Comparable::get()), "object");
 }
 
 TEST(OptionalTest, HashValue) {
   // Check that None, false, and true all hash differently.
   Optional<bool> B, B0 = false, B1 = true;
   EXPECT_NE(hash_value(B0), hash_value(B));
   EXPECT_NE(hash_value(B1), hash_value(B));
   EXPECT_NE(hash_value(B1), hash_value(B0));