[debuginfo-tests] Fix Dexter process creation failure on Windows

When writing the Windows dbgeng driver for Dexter, I couldn't work out why it
would either launch a process and leave it free running, or if I started the
process suspended, never do anything with it. The result was a hack to create
and attach processes manually. This has been flaking out on Reids Windows
buildbot, and clearly wasn't a good solution.

Digging into this, it turns out that the "normal" cdb / windbg behaviour of
breaking whenever we attach to a process is not the default: it has to be
explicitly requested from the debug engine. This patch does so (by setting
DEBUG_ENGOPT_INITIAL_BREAK in the engine options), after which we can simply
call "CreateProcessAndAttach2" and everything automagically works.

No test for this behaviour: everything was just broken before.

Differential Revision: https://reviews.llvm.org/D74409