Allow multiple simultaneous connections to platform.

Adds the --server argument to lldb-server platform which when specified will allow multiple simultaneous connections by forking off to handle each individual connection. This will allow us to run the remote tests in parallel.

Test Plan:
Run: lldb-server platform --listen *:1234 --server
Connect from multiple lldb clients simultaneously.
I will also test running the test suite remotely with multiple simultaneous jobs.

Differential Revision: http://reviews.llvm.org/D8452

llvm-svn: 233185