Fix for missing prompt on Windows

Summary: On Windows (and possibly other hosts with LLDB_DISABLE_LIBEDIT defined), the (lldb) prompt won't print after async output, like from a breakpoint hit or a step. This patch forces the prompt to be printed out after async output.

Reviewers: zturner, clayborg

Subscribers: amccarth, lldb-commits

Differential Revision: http://reviews.llvm.org/D18335

llvm-svn: 264332