Fix Initializer (and thus in particular SharedPtr) for immovable …

…types

before C++17. They cannot be returned by value.

To support `SharedPtr` for such types, add `Initializer::ConstructAt()` for
placement `new`. Also add `Initializer::MakeUnique()` for a common application
of placement `new`.

Use `Initializer::{MakeUnique,ConstructAt}()` in `{,Intrusive}SharedPtr` and
`Chain::Block`, so that they support immovable types also before C++17.

This in particular fixes `{Zstd,Lz4}Dictionary` before C++17.

Internal cleanup: do not specialize `Initializer`, choose the appropriate base
class instead. This reduces code duplication.

PiperOrigin-RevId: 669371179

riegeli/base/chain_details.h

@@ -25,7 +25,6 @@
 #include <iosfwd>
 #include <iterator>
 #include <memory>
-#include <new>
 #include <string>
 #include <type_traits>
 #include <utility>
@@ -533,7 +532,7 @@ void Chain::ExternalMethodsFor<T>::RegisterSubobjects(
 template <typename T>
 inline Chain::RawBlock::RawBlock(Initializer<T> object) {
   external_.methods = &ExternalMethodsFor<T>::kMethods;
-  new (&unchecked_external_object<T>()) T(std::move(object));
+  std::move(object).ConstructAt(&unchecked_external_object<T>());
   const absl::string_view data =
       riegeli::ToStringView(unchecked_external_object<T>());
   data_ = data.data();
@@ -547,7 +546,7 @@ inline Chain::RawBlock::RawBlock(Initializer<T> object,
                                  absl::string_view substr)
     : data_(substr.data()), size_(substr.size()) {
   external_.methods = &ExternalMethodsFor<T>::kMethods;
-  new (&unchecked_external_object<T>()) T(std::move(object));
+  std::move(object).ConstructAt(&unchecked_external_object<T>());
   RIEGELI_ASSERT(is_external()) << "A RawBlock with allocated_end_ == nullptr "
                                    "should be considered external";
 }

riegeli/base/external_ref_base.h

@@ -778,7 +778,7 @@ class ExternalRef {
   class ObjectForCordWhole {
    public:
     explicit ObjectForCordWhole(Initializer<T> object)
-        : ptr_(std::make_unique<T>(std::move(object))) {}
+        : ptr_(std::move(object).MakeUnique()) {}
 
     ObjectForCordWhole(ObjectForCordWhole&& that) = default;
     ObjectForCordWhole& operator=(ObjectForCordWhole&& that) = default;