[XRay] basic mode PID and TID always fetch

Summary: XRayRecords now includes a PID field. Basic handlers fetch pid and tid each time they are called instead of caching the value. Added a testcase that calls fork and checks if the child TID is different from the parent TID to verify that the processes' TID are different in the trace.

Reviewers: dberris, Maknee

Reviewed By: dberris, Maknee

Subscribers: kpw, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D49025

llvm-svn: 336769