async structured data packet handling improvements

This change does the following:
* Changes the signature for the continuation delegate method that handles
  async structured data from accepting an already-parsed structured data
  element to taking just the packet contents.
* Moves the conversion of the JSON-async: packet contents from
  GDBRemoteClientBase to the continuation delegate method.
* Adds a new unit test for verifying that the $JSON-asyc: packets get
  decoded and that the decoded packets get forwarded on to the delegate
  for further processing. Thanks to Pavel for making that whole section of
  code easily unit testable!
* Tightens up the packet verification on reception of a $JSON-async:
  packet contents. The code prior to this change is susceptible to a
  segfault if a packet is carefully crafted that starts with $J but
  has a total length shorter than the length of "$JSON-async:".

Reviewers: labath, clayborg, zturner

Differential Revision: https://reviews.llvm.org/D23884

llvm-svn: 281121