[XRay] Fix TSC and atomic custom/typed event accounting

Summary:
This is a follow-on change to D53858 which turns out to have had a TSC
accounting bug when writing out function exit records in FDR mode.

This change adds a number of tests to ensure that:

- We are handling the delta between the exit TSC and the last TSC we've
  seen.

- We are writing the custom event and typed event records as a single
  update to the buffer extents.

- We are able to catch boundary conditions when loading FDR logs.

We introduce a TSC matcher to the test helpers, which we use in the
testing/verification of the TSC accounting change.

Reviewers: mboerger

Subscribers: mgorny, hiraditya, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D53967

llvm-svn: 345905