[ADT] Deprecate llvm::Optional::transform

[kazu]

Summary

I've migrated all known users of llvm::Optional::transform. This
patch deprecates it to facilitate the migration from llvm::Optional to
std::optional.

[MaskRay]

I suspect very few if any project outside llvm-project uses this API, so removing it directly may be fine as well.

[kazu]

Decided to remove transform instead.

llvm/include/llvm/ADT/Optional.h

public:
   T *operator->() { return &Storage.value(); }
   constexpr const T &operator*() const & { return value(); }
   T &operator*() & { return value(); }
 
   template <typename U> constexpr T value_or(U &&alt) const & {
     return has_value() ? value() : std::forward<U>(alt);
   }
 
-  /// Apply a function to the value if present; otherwise return std::nullopt.
-  template <class Function>
-  auto transform(const Function &F) const & -> Optional<decltype(F(value()))> {
-    if (*this)
-      return F(value());
-    return std::nullopt;
-  }
-
   T &&value() && { return std::move(Storage.value()); }
   T &&operator*() && { return std::move(Storage.value()); }
 
   template <typename U> T value_or(U &&alt) && {
     return has_value() ? std::move(value()) : std::forward<U>(alt);
   }
-
-  /// Apply a function to the value if present; otherwise return std::nullopt.
-  template <class Function>
-  auto transform(
-      const Function &F) && -> Optional<decltype(F(std::move(*this).value()))> {
-    if (*this)
-      return F(std::move(*this).value());
-    return std::nullopt;
-  }
 };
 
 template<typename T>
 Optional(const T&) -> Optional<T>;
 
 template <class T> llvm::hash_code hash_value(const Optional<T> &O) {
   return O ? hash_combine(true, *O) : hash_value(false);
 }

llvm/unittests/ADT/OptionalTest.cpp

TEST(OptionalTest, MoveValueOr) {
   A = MoveOnly(5);
   MoveOnly::ResetCounts();
   EXPECT_EQ(5, std::move(A).value_or(MoveOnly(42)).val);
   EXPECT_EQ(1u, MoveOnly::MoveConstructions);
   EXPECT_EQ(0u, MoveOnly::MoveAssignments);
   EXPECT_EQ(2u, MoveOnly::Destructions);
 }
 
-TEST(OptionalTest, Transform) {
-  Optional<int> A;
-
-  Optional<int> B = A.transform([&](int N) { return N + 1; });
-  EXPECT_FALSE(B.has_value());
-
-  A = 3;
-  Optional<int> C = A.transform([&](int N) { return N + 1; });
-  EXPECT_TRUE(C.has_value());
-  EXPECT_EQ(4, C.value());
-}
-
-TEST(OptionalTest, MoveTransform) {
-  Optional<MoveOnly> A;
-
-  MoveOnly::ResetCounts();
-  Optional<int> B =
-      std::move(A).transform([&](const MoveOnly &M) { return M.val + 2; });
-  EXPECT_FALSE(B.has_value());
-  EXPECT_EQ(0u, MoveOnly::MoveConstructions);
-  EXPECT_EQ(0u, MoveOnly::MoveAssignments);
-  EXPECT_EQ(0u, MoveOnly::Destructions);
-
-  A = MoveOnly(5);
-  MoveOnly::ResetCounts();
-  Optional<int> C =
-      std::move(A).transform([&](const MoveOnly &M) { return M.val + 2; });
-  EXPECT_TRUE(C.has_value());
-  EXPECT_EQ(7, C.value());
-  EXPECT_EQ(0u, MoveOnly::MoveConstructions);
-  EXPECT_EQ(0u, MoveOnly::MoveAssignments);
-  EXPECT_EQ(0u, MoveOnly::Destructions);
-}
-
 struct EqualTo {
   template <typename T, typename U> static bool apply(const T &X, const U &Y) {
     return X == Y;
   }
 };
 
 struct NotEqualTo {
   template <typename T, typename U> static bool apply(const T &X, const U &Y) {