[ADT] Backport std::to_underlying from C++23 (llvm#70681)

This patch backports a one-liner `std::to_underlying` that came with C++23. This is useful for refactoring unscoped enums into scoped enums, because the latter are not implicitly convertible to integer types.

I followed libc++ implementation, but I consider their testing too heavy for us, so I wrote a simpler set of tests.

llvm/include/llvm/ADT/STLForwardCompat.h

@@ -60,6 +60,13 @@ auto transformOptional(std::optional<T> &&O, const Function &F)
   return std::nullopt;
 }
 
+/// Returns underlying integer value of an enum. Backport of C++23
+/// std::to_underlying.
+template <typename Enum>
+[[nodiscard]] constexpr std::underlying_type_t<Enum> to_underlying(Enum E) {
+  return static_cast<std::underlying_type_t<Enum>>(E);
+}
+
 } // namespace llvm
 
 #endif // LLVM_ADT_STLFORWARDCOMPAT_H

llvm/unittests/ADT/STLForwardCompatTest.cpp

@@ -119,4 +119,21 @@ TEST(TransformTest, MoveTransformLlvm) {
   EXPECT_EQ(0u, MoveOnly::Destructions);
 }
 
+TEST(TransformTest, ToUnderlying) {
+  enum E { A1 = 0, B1 = -1 };
+  static_assert(llvm::to_underlying(A1) == 0);
+  static_assert(llvm::to_underlying(B1) == -1);
+
+  enum E2 : unsigned char { A2 = 0, B2 };
+  static_assert(
+      std::is_same_v<unsigned char, decltype(llvm::to_underlying(A2))>);
+  static_assert(llvm::to_underlying(A2) == 0);
+  static_assert(llvm::to_underlying(B2) == 1);
+
+  enum class E3 { A3 = -1, B3 };
+  static_assert(std::is_same_v<int, decltype(llvm::to_underlying(E3::A3))>);
+  static_assert(llvm::to_underlying(E3::A3) == -1);
+  static_assert(llvm::to_underlying(E3::B3) == 0);
+}
+
 } // namespace