llgs: implement qThreadStopInfo.

This change implements this ticket:
http://llvm.org/bugs/show_bug.cgi?id=20899

Adds the qThreadStopInfo RSP command for llgs and includes a test that
verifies both debugserver and llgs respond with something reasonable
on a multithreaded app.
Avoid a couple of assertions when preprocessing with modules

1. We were hitting the NextIsPrevious assertion because we were trying
to merge decl chains that were independent of each other because we had
no Sema object to allow them to find existing decls. This is fixed by
delaying loading the "preloaded" decls until Sema is available.

2. We were trying to get identifier info from an annotation token, which
asserts.  The fix is to special-case the module annotations in the
preprocessed output printer.

Fixed in a single commit because when you hit 1 you almost invariably
hit 2 as well.