Make RocksDB codebase compatible with newer compilers like clang-12 (#9370)

Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9370

GCC and newer clang, e.g. clang-12 treat `std::unique_ptr` slightly differently.
For the following code
```
#include <iostream>
#include <memory>
#include <type_traits>

struct A {
    std::unique_ptr<int> m1;
};

int main()
{
    std::cout << std::boolalpha;
    std::cout << std::is_standard_layout<A>::value << '\n';
    return 0;
}
```
GCC11(C++20) (tested on https://en.cppreference.com/w/cpp/types/is_standard_layout) will print "true", while newer clang, e.g. clang-12 will print "false". This breaks the usage of `offsetof()` on structs with non-static members of type `std::unique_ptr`.
Fixing this by replacing the builtin `offsetof` with a trick documented at https://gist.github.com/graphitemaster/494f21190bb2c63c5516.

Reviewed By: jay-zhuang

Differential Revision: D33420840

fbshipit-source-id: 02bde281dfa28809bec787ad0f7019e85dd9c607

include/rocksdb/comparator.h

@@ -75,7 +75,7 @@ class Comparator : public Customizable {
   //
   // Names starting with "rocksdb." are reserved and should not be used
   // by any clients of this package.
-  virtual const char* Name() const override = 0;
+  const char* Name() const override = 0;
 
   // Advanced functions: these are used to reduce the space requirements
   // for internal data structures like index blocks.

include/rocksdb/file_checksum.h

@@ -85,7 +85,7 @@ class FileChecksumGenFactory : public Customizable {
       const FileChecksumGenContext& context) = 0;
 
   // Return the name of this FileChecksumGenFactory.
-  virtual const char* Name() const override = 0;
+  const char* Name() const override = 0;
 };
 
 // FileChecksumList stores the checksum information of a list of files (e.g.,

include/rocksdb/memtablerep.h

@@ -311,7 +311,7 @@ class MemTableRepFactory : public Customizable {
     return CreateMemTableRep(key_cmp, allocator, slice_transform, logger);
   }
 
-  virtual const char* Name() const override = 0;
+  const char* Name() const override = 0;
 
   // Return true if the current MemTableRep supports concurrent inserts
   // Default: false

include/rocksdb/sst_partitioner.h

@@ -93,7 +93,7 @@ class SstPartitionerFactory : public Customizable {
       const SstPartitioner::Context& context) const = 0;
 
   // Returns a name that identifies this partitioner factory.
-  virtual const char* Name() const override = 0;
+  const char* Name() const override = 0;
 };
 
 /*

include/rocksdb/table_properties.h

@@ -162,7 +162,7 @@ class TablePropertiesCollectorFactory : public Customizable {
       TablePropertiesCollectorFactory::Context context) = 0;
 
   // The name of the properties collector can be used for debugging purpose.
-  virtual const char* Name() const override = 0;
+  const char* Name() const override = 0;
 
   // Can be overridden by sub-classes to return the Name, followed by
   // configuration info that will // be logged to the info log when the

options/customizable_test.cc

@@ -206,22 +206,31 @@ struct SimpleOptions {
   TestCustomizable* cp = nullptr;
 };
 
+static SimpleOptions dummy_simple_options;
+template <typename T1>
+int offset_of(T1 SimpleOptions::*member) {
+  return static_cast<int>(
+      reinterpret_cast<uintptr_t>(
+          std::addressof(dummy_simple_options.*member)) -
+      reinterpret_cast<uintptr_t>(std::addressof(dummy_simple_options)));
+}
+
 static std::unordered_map<std::string, OptionTypeInfo> simple_option_info = {
 #ifndef ROCKSDB_LITE
     {"bool",
-     {offsetof(struct SimpleOptions, b), OptionType::kBoolean,
+     {offset_of(&SimpleOptions::b), OptionType::kBoolean,
       OptionVerificationType::kNormal, OptionTypeFlags::kNone}},
     {"unique",
      OptionTypeInfo::AsCustomUniquePtr<TestCustomizable>(
-         offsetof(struct SimpleOptions, cu), OptionVerificationType::kNormal,
+         offset_of(&SimpleOptions::cu), OptionVerificationType::kNormal,
          OptionTypeFlags::kAllowNull)},
     {"shared",
      OptionTypeInfo::AsCustomSharedPtr<TestCustomizable>(
-         offsetof(struct SimpleOptions, cs), OptionVerificationType::kNormal,
+         offset_of(&SimpleOptions::cs), OptionVerificationType::kNormal,
          OptionTypeFlags::kAllowNull)},
     {"pointer",
      OptionTypeInfo::AsCustomRawPtr<TestCustomizable>(
-         offsetof(struct SimpleOptions, cp), OptionVerificationType::kNormal,
+         offset_of(&SimpleOptions::cp), OptionVerificationType::kNormal,
          OptionTypeFlags::kAllowNull)},
 #endif  // ROCKSDB_LITE
 };