Added setting: platform.plugin.linux.use-llgs-for-local (boolean), defaults to false

This setting is not yet hooked up to anything.  In preparation for
adding/testing llgs for local Linux debugging.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216690 91177308-0d34-0410-b5e6-96231b3b80d8