[XRay] Add a RecordPrinter visitor for FDR Records

Summary:
This change adds a `RecordPrinter` type which does some basic text
serialization of the FDR record instances. This is one component of the
tool we're building to dump the records from an FDR mode log as-is.

This is a small part of D50441.

Reviewers: eizan, kpw

Subscribers: mgorny, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D51672

llvm-svn: 341447