Fixed a race condition where the prompt would randomly go missing.

el_gets was using fflush to output it's string, but because we have our own filter running on the piped pty output, fflush only causes the prompt to be written into the pipe, and does not cause the filter code to run immediately.

The simplest fix is to manually block and wait for all editline output to be processed.

This fixes PR 14637.

llvm-svn: 191392