Implement handling of `library:` keys in thread stop replies.

Summary:
When a windows remote stops because of a DLL load/unload, the debug server
sends a stop reply packet that contains a `library` key with any value (usually
just `library:1`). This indicates to the debugger that a library has been
loaded or unloaded and that the list of libraries should be refreshed (usually
with `qXfer:libraries:read`).

This change just triggers a call to `LoadModules()` which in turns will send a
remote library read command when a stop reply that requests it is received.

Reviewers: clayborg, zturner, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12218

llvm-svn: 245708