posix_spawn should retry upon EINTR

Summary:
We've seen cases of bots failing with:
  clang: error: unable to execute command: posix_spawn failed: Interrupted system call

Add a small retry loop to posix_spawn in case this happens. Don't retry too much in case there's some systemic problem going on, but retry a few times.
<rdar://problem/50181448>

Reviewers: Bigcheese, arphaman

Subscribers: jkorous, dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61096

llvm-svn: 359152