Support for debugging against a remote stock gdbserver.

There are a couple of pieces:
 * some lazy-evaluation members that store info listed in a qSupported response
 * new method SendPacketsAndConcatenateResponses which is used for
   fetching fixed-size objects from the remote gdbserver by using multiple
   packets if necessary (first use will be to fetch shared-library XML files).

llvm-svn: 200072