[Bitstream] Use alignTo to make code more readable. NFC

I was recently debugging a similar issue to https://reviews.llvm.org/D86500 only with a large metadata section. Only after I finished debugging it did I discover it was fixed very recently.

My version of the fix was going to alignTo since that uses uint64_t and improves the readability of the code. So I though I would go ahead and share it.

Differential Revision: https://reviews.llvm.org/D86957