[llvm] Deprecate llvm::empty (NFC)

[kazu]

Summary

This patch deprecates llvm::empty as I've migrated all known uses of
llvm::empty(x) to x.empty().

[MaskRay]

Thanks!

[kazu]

In D134141#3798733, @MaskRay wrote:

Thanks!

Thanks for the review!

llvm/include/llvm/ADT/STLExtras.h

 template <typename T>
 void adl_swap(T &&lhs, T &&rhs) noexcept(
     noexcept(adl_detail::adl_swap(std::declval<T>(), std::declval<T>()))) {
   adl_detail::adl_swap(std::forward<T>(lhs), std::forward<T>(rhs));
 }
 
 /// Test whether \p RangeOrContainer is empty. Similar to C++17 std::empty.
 template <typename T>
+LLVM_DEPRECATED("Use x.empty() instead", "empty")
 constexpr bool empty(const T &RangeOrContainer) {
   return adl_begin(RangeOrContainer) == adl_end(RangeOrContainer);
 }
 
 /// Returns true if the given container only contains a single element.
 template <typename ContainerTy> bool hasSingleElement(ContainerTy &&C) {
   auto B = std::begin(C), E = std::end(C);
   return B != E && std::next(B) == E;

llvm/unittests/ADT/STLExtrasTest.cpp

TEST(STLExtrasTest, ADLTest) {
   EXPECT_EQ(s.swap_val, "lhs");
   EXPECT_EQ(s2.swap_val, "rhs");
 
   int count = 0;
   llvm::for_each(s, [&count](int) { ++count; });
   EXPECT_EQ(5, count);
 }
 
-TEST(STLExtrasTest, EmptyTest) {
-  std::vector<void*> V;
-  EXPECT_TRUE(llvm::empty(V));
-  V.push_back(nullptr);
-  EXPECT_FALSE(llvm::empty(V));
-
-  std::initializer_list<int> E = {};
-  std::initializer_list<int> NotE = {7, 13, 42};
-  EXPECT_TRUE(llvm::empty(E));
-  EXPECT_FALSE(llvm::empty(NotE));
-
-  auto R0 = make_range(V.begin(), V.begin());
-  EXPECT_TRUE(llvm::empty(R0));
-  auto R1 = make_range(V.begin(), V.end());
-  EXPECT_FALSE(llvm::empty(R1));
-}
-
 TEST(STLExtrasTest, DropBeginTest) {
   SmallVector<int, 5> vec{0, 1, 2, 3, 4};
 
   for (int n = 0; n < 5; ++n) {
     int i = n;
     for (auto &v : drop_begin(vec, n)) {
       EXPECT_EQ(v, i);
       i += 1;