APINotes: add property models for YAML attributes

This adds internal representation of the attributes in a more usable
form. This is meant to allow programmatic access to the attributes that
are specified in the YAML data.

This is based upon the work contributed by Apple at
https://github.com/llvm/llvm-project-staging/tree/staging/swift/apinotes.

Differential Revision: https://reviews.llvm.org/D91104
Reviewed By: Gabor Marton