[llvm][Object] Add missing const qualifier for value_type in content_iterator.

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

Author: Dmitry Bushev

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 &;