Occasionally, during test teardown, LLDB writes to a closed pipe.
Sometimes the communication is inherently unreliable, so LLDB tries to
avoid being killed due to SIGPIPE. Actually, it explicitly calls
signal(SIGPIPE, SIG_IGN). However, LLVM's default SIGPIPE behavior is
to exit with IO_ERR. Opt LLDB out of that.
I expect that this will resolve some LLDB test suite flakiness (tests
randomly failing with IO_ERR) that we've seen since r344372.