Use socketpair on all Unix platforms

Using TCP sockets is insecure against local attackers, and possibly
against remote attackers too (some vulnerabilities may allow tricking a
browser to make a request to localhost). Use socketpair (which is immune
to such attacks) on all Unix platforms.

Patch by Demi Marie Obenour < demiobenour@gmail.com >

Differential Revision: https://reviews.llvm.org/D33213

llvm-svn: 314127