Add wait for child processe(s) to exit. (amended+clang-formatted)

It was possible for the parent process to exit before the
forked child process had finished. In some shells, this
causes the pipe to close and FileCheck misses some output
from the child. Waiting for the child process to exit before
exiting the parent, assures that all output from stdout and
stderr is combined and forwarded through the pipe to FileCheck.

rdar://95241490

Differential Revision: https://reviews.llvm.org/D128565