[llvm][Object] Add missing const qualifier for value_type in content_iterator (#124106)

value_type was defined as non-const for content_iterator, although it's
methods returned a const pointers/references. This prevented it from
using in some algorithms from STLExtras.h

llvm/include/llvm/Object/SymbolicFile.h

@@ -71,7 +71,7 @@ template <class content_type> class content_iterator {
 
 public:
   using iterator_category = std::forward_iterator_tag;
-  using value_type = content_type;
+  using value_type = const content_type;
   using difference_type = std::ptrdiff_t;
   using pointer = value_type *;
   using reference = value_type &;

llvm/unittests/Object/SymbolicFileTest.cpp

@@ -7,8 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Object/SymbolicFile.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TargetParser/Host.h"
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include <sstream>
 
@@ -38,3 +40,22 @@ TEST(Object, DataRefImplOstream) {
 
   EXPECT_EQ(Expected, s);
 }
+
+struct ProxyContent {
+  unsigned Index = 0;
+  ProxyContent(unsigned Index) : Index(Index) {};
+  void moveNext() { ++Index; }
+
+  bool operator==(const ProxyContent &Another) const {
+    return Index == Another.Index;
+  }
+};
+
+TEST(Object, ContentIterator) {
+  using Iter = llvm::object::content_iterator<ProxyContent>;
+  auto Sequence = llvm::make_range(Iter(0u), Iter(10u));
+  auto EvenSequence = llvm::make_filter_range(
+      Sequence, [](auto &&PC) { return PC.Index % 2 == 0; });
+
+  EXPECT_THAT(EvenSequence, testing::ElementsAre(0u, 2u, 4u, 6u, 8u));
+}