[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