[llvm-exegesis][NFC] Rewrite of the YAML serialization.

[llvm-exegesis][NFC] Rewrite of the YAML serialization.

Summary: This is a NFC in preparation of exporting the initial registers as part of the YAML dump

Reviewers: courbet

Reviewed By: courbet

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D52427

llvm-svn: 342967