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

[Jakub Kuderski via llvm-commits]

================
@@ -38,3 +40,24 @@ TEST(Object, DataRefImplOstream) {
 
   EXPECT_EQ(Expected, s);
 }
+
+struct ProxyContent {
+  unsigned Index = 0;
+  ProxyContent(unsigned Index) : Index(Index) {};
+  void moveNext() { Index++; }
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kuhar wrote:

```suggestion
  void moveNext() { ++Index; }
```
see https://llvm.org/docs/CodingStandards.html#prefer-preincrement

https://github.com/llvm/llvm-project/pull/124106