[ADT] Implement ArrayRef::operator< and other comparisons (#147277)

Order ArrayRefs using std::lexicographical_compare, just like
std::vector and SmallVector.

Author: jayfoad

llvm/include/llvm/ADT/ArrayRef.h

@@ -565,6 +565,27 @@ namespace llvm {
     return !(LHS == RHS);
   }
 
+  template <typename T>
+  inline bool operator<(ArrayRef<T> LHS, ArrayRef<T> RHS) {
+    return std::lexicographical_compare(LHS.begin(), LHS.end(), RHS.begin(),
+                                        RHS.end());
+  }
+
+  template <typename T>
+  inline bool operator>(ArrayRef<T> LHS, ArrayRef<T> RHS) {
+    return RHS < LHS;
+  }
+
+  template <typename T>
+  inline bool operator<=(ArrayRef<T> LHS, ArrayRef<T> RHS) {
+    return !(LHS > RHS);
+  }
+
+  template <typename T>
+  inline bool operator>=(ArrayRef<T> LHS, ArrayRef<T> RHS) {
+    return !(LHS < RHS);
+  }
+
   /// @}
 
   template <typename T> hash_code hash_value(ArrayRef<T> S) {

llvm/unittests/ADT/ArrayRefTest.cpp

@@ -231,6 +231,27 @@ TEST(ArrayRefTest, EmptyEquals) {
   EXPECT_TRUE(ArrayRef<unsigned>() == ArrayRef<unsigned>());
 }
 
+TEST(ArrayRefTest, Compare) {
+  ArrayRef<char> Ban("Ban");
+  ArrayRef<char> Banana("Banana");
+  ArrayRef<char> Band("Band");
+
+  EXPECT_TRUE(Ban < Banana);
+  EXPECT_TRUE(Ban <= Banana);
+  EXPECT_FALSE(Ban > Banana);
+  EXPECT_FALSE(Ban >= Banana);
+
+  EXPECT_FALSE(Banana < Banana);
+  EXPECT_TRUE(Banana <= Banana);
+  EXPECT_FALSE(Banana > Banana);
+  EXPECT_TRUE(Banana >= Banana);
+
+  EXPECT_TRUE(Banana < Band);
+  EXPECT_TRUE(Banana <= Band);
+  EXPECT_FALSE(Banana > Band);
+  EXPECT_FALSE(Banana >= Band);
+}
+
 TEST(ArrayRefTest, ConstConvert) {
   int buf[4];
   for (int i = 0; i < 4; ++i)