[NFC][ADT] Add reverse iterators and value_type to StringRef

- Add reverse iterators and `value_type` to StringRef.
- Add unit test for all 4 iterator flavors.
- This prepares StringRef to be used with `SequenceToOffsetTable`.

Author: jurahul

llvm/include/llvm/ADT/StringRef.h

@@ -17,6 +17,7 @@
 #include <cassert>
 #include <cstddef>
 #include <cstring>
+#include <iterator>
 #include <limits>
 #include <string>
 #include <string_view>
@@ -54,6 +55,9 @@ namespace llvm {
     using iterator = const char *;
     using const_iterator = const char *;
     using size_type = size_t;
+    using value_type = char;
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
   private:
     /// The start of the string, in an external buffer.
@@ -112,6 +116,14 @@ namespace llvm {
 
     iterator end() const { return Data + Length; }
 
+    reverse_iterator rbegin() const {
+      return std::make_reverse_iterator(end());
+    }
+
+    reverse_iterator rend() const {
+      return std::make_reverse_iterator(begin());
+    }
+
     const unsigned char *bytes_begin() const {
       return reinterpret_cast<const unsigned char *>(begin());
     }

llvm/unittests/ADT/StringRefTest.cpp

@@ -72,9 +72,18 @@ TEST(StringRefTest, EmptyInitializerList) {
 
 TEST(StringRefTest, Iteration) {
   StringRef S("hello");
-  const char *p = "hello";
-  for (const char *it = S.begin(), *ie = S.end(); it != ie; ++it, ++p)
-    EXPECT_EQ(*it, *p);
+  constexpr StringLiteral CS("hello");
+
+  // Note: Cannot use literal strings in equal() as iteration over a literal
+  // string includes the null terminator.
+  constexpr std::string_view RefFwd("hello");
+  constexpr std::string_view RefRev("olleh");
+
+  EXPECT_TRUE(equal(S, RefFwd));
+  EXPECT_TRUE(equal(CS, RefFwd));
+  // reverse() builds an iterator range using StringRef::rbegin()/rend().
+  EXPECT_TRUE(equal(reverse(S), RefRev));
+  EXPECT_TRUE(equal(reverse(CS), RefRev));
 }
 
 TEST(StringRefTest, StringOps) {