[clangd] Use our own relation kind.

Summary:
Move the RelationKind from Serialization.h to Relation.h. This patch doesn't
introduce any breaking changes.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68981

llvm-svn: 375117