[NFC] [Serialization] Add static assert for Num*Declbits

This re-commits part of c79635cc. It is reverted since it contains
platform-inconsistent constant. Now the patch only contains constant
defined in DeclBase.h so it should be platform-independent. And this
should be still helpful.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D141992