[XRay] Improve error reporting when loading traces

Summary:
This change uses a single offset pointer used throughout the
implementation of the individual record parsers. This allows us to
report where in a trace file parsing failed.

We're still in an intermediate step here as we prepare to refactor this
further into a set of types and use object-oriented design principles
for a cleaner implementation. The next steps will be to allow us to
parse/dump files in a streaming fashion and incrementally build up the
structures in memory instead of the current all-or-nothing approach.

Reviewers: kpw, eizan

Reviewed By: kpw

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D50169

llvm-svn: 339092