Implement WindowsDYLD::DidAttach for use with gdb-server attach

Summary:
Windows lldb debugging currently uses a process plugin to handle
launching and attaching to a process. Launching a process via a debug
server (e.g. ds2) and attaching to it with `gdb-remote port` currently
doesn't communicate address information of the executable properly.

Implement DynamicLoaderWindowsDYLD::DidAttach which allow us to
obtain the proper executable load address.

Differential Revision: https://reviews.llvm.org/D55383

llvm-svn: 348526