Update LLDB to use LLVM's DynamicLibrary.

LLDB had implemented its own DynamicLibrary class for plugin
support.  LLVM has an equivalent mechanism, so this patch deletes
the duplicated code in LLDB and updates LLDB to reference the
mechanism provided by LLVM.
Create a HostProcess class.

This is a lightweight wrapper around a pid.  It is intended to be
lightweight enough to serve as a replacement anywhere we currently
store a pid.  It provides convenience methods and common process
operations.

This patch does not yet make use of HostProcess anywhere.