Fix IPv6 support on lldb-server platform

This is a general fix for the ConnectionFileDescriptor class but my main
motivation was to make lldb-server working with IPv6.
The connect URI can use square brackets ([]) to wrap the interface part
of the URI (e.g.: <scheme>://[<interface>]:<port>). For IPv6 addresses
this is a must since its ip can include colons and it will overlap with
the port colon otherwise. The URIParser class parses the square brackets
correctly but the ConnectionFileDescriptor doesn't generate them for
IPv6 addresses making it impossible to connect to the gdb server when
using this protocol.

How to reproduce the issue:

$ lldb-server p --server --listen [::1]:8080
...
$ lldb
(lldb) platform select remote-macosx
(lldb) platform connect connect://[::1]:8080
(lldb) platform process -p <pid>
error: unable to launch a GDB server on 'computer'

The server was actually launched we were just not able to connect to it.
With this fix lldb will correctly connect. I fixed this by wrapping the
ip portion with [].

Differential Revision: https://reviews.llvm.org/D61833

Patch by António Afonso <antonio.afonso@gmail.com>

llvm-svn: 361079