folly:: singleton: ubsan: fix calling overriden method from ctor

folly:: singleton: ubsan: fix calling overriden method from ctor

Summary:
Ubsan complains when overriden method is called from ctor:
```
0x000000b04d40: note: object is of type 'folly::detail::SingletonHolder<XYZ>' 00 00 00 00  08 50 c3 8d da 7f 00 00  e8 4f c3 8d da 7f 00 00  18 0d 4c b4 da 7f 00 00  a0 16 90 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'folly::detail::SingletonHolder<XYZ>'
```

called from `folly::SingletonVault::registerSingleton(folly::detail::SingletonHolderBase*)`

Reviewed By: ot

Differential Revision: D3462037

fbshipit-source-id: 6df283dd53df42d5d318990f60aba370ceed6395