Add const versions of begin/end

Summary:
`ConcurrentHashMap` provides `cbegin` and `cend`, but they are not suitable for range-based loops.
This diff adds const versions of `begin` and `end` that are required for range-based loops.

Reviewed By: yfeldblum

Differential Revision: D8131142

fbshipit-source-id: b3af164d403c86ddfe4d27aaca293fbc4e2bb9d2

folly/concurrency/ConcurrentHashMap.h

@@ -194,6 +194,14 @@ class ConcurrentHashMap {
     return ConstIterator(this);
   }
 
+  ConstIterator end() const noexcept {
+    return cend();
+  }
+
+  ConstIterator begin() const noexcept {
+    return cbegin();
+  }
+
   std::pair<ConstIterator, bool> insert(
       std::pair<key_type, mapped_type>&& foo) {
     auto segment = pickSegment(foo.first);

folly/concurrency/test/ConcurrentHashMapTest.cpp

@@ -696,3 +696,15 @@ TEST(ConcurrentHashMap, DeletionMultipleMaps) {
   EXPECT_TRUE(del1);
   EXPECT_TRUE(del2);
 }
+
+TEST(ConcurrentHashMap, ForEachLoop) {
+  ConcurrentHashMap<int, int> map;
+  map.insert(1, 2);
+  size_t iters = 0;
+  for (const auto& kv : map) {
+    EXPECT_EQ(kv.first, 1);
+    EXPECT_EQ(kv.second, 2);
+    ++iters;
+  }
+  EXPECT_EQ(iters, 1);
+}