[XRay] Add a BlockVerifier visitor for FDR Records

Summary:
This patch implements a `BlockVerifier` type which enforces the
invariants of the log structure of FDR mode logs on a per-block basis.
This ensures that the data we encounter from an FDR mode log
semantically correct (i.e. that records follow the documented "grammar"
for FDR mode log records).

This is another part of the refactoring of D50441.

Reviewers: mboerger, eizan

Subscribers: mgorny, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D51723

llvm-svn: 341628