This adds the skeleton of the YAML Compiler for APINotes. This change
only adds the YAML IO model for the API Notes along with a new testing
tool `apinotes-test` which can be used to verify that can round trip the
YAML content properly. It provides the basis for the future work which
will add a binary serialization and deserialization format to the data
model.
This is based on the code contributed by Apple at
https://github.com/llvm/llvm-project-staging/tree/staging/swift/apinotes.
Differential Revision: https://reviews.llvm.org/D88859
Reviewed By: Gabor Marton
Nico Weber