[XRay] Add a BlockIndexer visitor for FDR Records.

Summary:
This change adds a `BlockIndexer` type which maintains pointers to
records that belong to the same process+thread pairs. The indexing
happens with order of appearance of records as they are visited.

This version of the indexer currently only supports FDR version 3 logs,
which contain `BufferExtent` records. We will add support for v2 and v1
logs in follow-up patches.

This is another part of D50441.

Reviewers: eizan, kpw, mboerger

Reviewed By: mboerger

Subscribers: mboerger, mgorny, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D51673

llvm-svn: 341518