[clangd] Serialization support for RelationSlab

Summary: This builds on D59407 to provide YAML and RIFF serialization support.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62459

llvm-svn: 362353