[XRay] Add the `llvm-xray fdr-dump` implementation

Summary:
In this change, we implement a `BlockPrinter` which orders records in a
Block that's been indexed by the `BlockIndexer`. This is used in the
`llvm-xray fdr-dump` tool which ties together the various types and
utilities we've been working on, to allow for inspection of XRay FDR
mode traces both with and without verification.

This change is the final step of the refactoring of D50441.

Reviewers: mboerger, eizan

Subscribers: mgorny, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D51846

llvm-svn: 341887