lldb can interrupt waitpid, so EINTR shouldn't be an error. This fixes the case
where there is no timeout. In the case where there is a timeout though, the
code is still wrong since it doesn't check that the alarm really went off.

Without this patch, I cannot debug a program that forks itself using
sys::ExecuteAndWait with lldb.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211918 91177308-0d34-0410-b5e6-96231b3b80d8