Increate backlog of lldb-platform's listener socket

lldb-platform's listener socket only had a backlog of one connection.
That means that if more than one client connected simultaneously, the
connection would be refused. The test suite can be run remotely with
dozens of threads connecting simultaneously. Raised this limit to 100
to effectively eliminate lost connections.

Test Plan:
run tests against a remote target

Differential Revision: http://reviews.llvm.org/D8696

llvm-svn: 233652