[ADT] Fix SmallVector unused template instantiation on 32-bit systems.

Summary:
This was introduced in dda3c19a aka D77621.

The unused template instantiation causes a warning on 32 bit systems
about truncating a uint64_t to 32-bit size_t.

Reviewed By: dblaikie, smeenai

Differential Revision: https://reviews.llvm.org/D79214