Use only unnamed pipes to launch lldb-server gdbserver.

Summary:
If we used unnamed pipes instead of named pipes, we can avoid having the
file system littered with debugserver-named-pipes if lldb-server happens to
crash for whatever reason. Also, on some buggy systems, it's possible to be
able to create but not to delete a fifo. Ideally, support for unnamed pipes
should be added to debugserver as well, so we can avoid the `#ifdef` here.

Reviewers: clayborg, vharron, chying

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11609

llvm-svn: 243667