[XRay][tools] Support tail-call exits before we write them in the runtime

Summary:
This change adds support for explicit tail-exit records to be written by
the XRay runtime. This lets us differentiate the tail exit
records/events in the log, and allows us to treat those exit events
especially in the future. For now we allow printing those out in YAML
(and reading them in).

Reviewers: kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37964

llvm-svn: 313514