Properly unload modules from target image list when using svr4 packets

Summary:
When we receive an svr4 packet from the remote, we check for new modules
and add them to the list of images in the target. However, we did not
do the same for modules which have been removed.

This was causing TestLoadUnload to fail when using ds2, which uses
svr4 packets to communicate all library info on Linux. This patch fixes
the failing test.

Reviewers: zturner, tfiala, ADodds

Subscribers: lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D19230

This is a cherry-pick of 267467

llvm-svn: 270133