Bitcode: add some convenience helpers for streaming bitcode

This adds a set of metaprogramming helpers to help define records and
serialize them out.  This is motivated by API Notes which use the
bitcode format to serialize out a binary representation of the data.
These helpers are generically useful though and could help simplify some
of the existing bitcode consumers as well.

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/D88582