Remove scheduling dependency from XRay :: Posix/fork_basic_logging.cc

Summary:
We've been seeing intermittent failures on our internal bots and we suspect
this may be due to the OS scheduling the child process to run before the parent
process.

This version ensures that the parent and child can be run in either order.

Reviewers: Maknee, dberris

Reviewed By: dberris

Subscribers: delcypher, #sanitizers, Maknee, llvm-commits

Differential Revision: https://reviews.llvm.org/D49501

llvm-svn: 337432