More fixes related to counting threads on Windows.

The Windows 10 loader spawns threads at startup, so
tests which count threads or assume that a given user
thread will be at a specific index are incorrect in
this case.  The fix here is to use the standard mechanisms
for getting the stopped thread (which is all we are
really interested in anyway) and correlating them with
the breakpoints that were set, and doing checks against
those things.

This fixes about 6 tests on Windows 10.

llvm-svn: 258586