[libc][fixedvector] Add const_iterator begin/end (llvm#96714)

Author: PiJoules

libc/src/__support/fixedvector.h

@@ -90,6 +90,11 @@ template <typename T, size_t CAPACITY> class FixedVector {
 
   LIBC_INLINE constexpr iterator begin() { return store.begin(); }
   LIBC_INLINE constexpr iterator end() { return iterator{&store[item_count]}; }
+
+  LIBC_INLINE constexpr const_iterator begin() const { return store.begin(); }
+  LIBC_INLINE constexpr const_iterator end() const {
+    return const_iterator{&store[item_count]};
+  }
 };
 
 } // namespace LIBC_NAMESPACE

libc/test/src/__support/fixedvector_test.cpp

@@ -96,3 +96,13 @@ TEST(LlvmLibcFixedVectorTest, ForwardIteration) {
     ASSERT_EQ(*it, arr[idx]);
   }
 }
+
+TEST(LlvmLibcFixedVectorTest, ConstForwardIteration) {
+  const LIBC_NAMESPACE::cpp::array<int, 4> arr{1, 2, 3, 4};
+  const LIBC_NAMESPACE::FixedVector<int, 5> vec(arr.begin(), arr.end());
+  ASSERT_EQ(vec.size(), arr.size());
+  for (auto it = vec.begin(); it != vec.end(); ++it) {
+    auto idx = it - vec.begin();
+    ASSERT_EQ(*it, arr[idx]);
+  }
+}