[BOLT][Instrumentation] Keep profile open in WatchProcess

When a binary is instrumented with --instrumentation-sleep-time and
instrumentation-wait-forks options and lauched, the profile is
periodically written until all the forks die. The problem is that we
cannot wait for the whole process tree, and we have no way to tell when
it's safe to read the profile. Hovewer, if we keep profile open
throughout the life of the process tree, we can use fuser to determine
when writing is finished.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D154436

(cherry picked from commit a7992981)