APINotes: add bitcode format schema definitions

This adds the bitcode format schema required for serialization of the
YAML data to a binary format.  APINotes are pre-compiled and re-used in
the binary format from the frontend.  These definitions provide the data
layout representation enabling writing (and eventually) reading of the
data in bitcode format.

This is extracted from the code contributed by Apple at
https://github.com/llvm/llvm-project-staging/tree/staging/swift/apinotes.

Differential Revision: https://reviews.llvm.org/D91997
Reviewed By: Gabor Marton