handle synthetic frames produced by GDB FrameFilter without a level

[tzcnt]

Synthetic frames produced by GDB's FrameFilter, e.g. async stack backtraces for C++20 coroutines produced by a GDB script do not have a Level, and cannot be assigned a Level with the current version of GDB. Attempting to set a breakpoint inside of an async stack then causes an error when walking the synthetic backtrace:

ERROR: Error while trying to enter break state. Debugging will now stop. Unrecognized format of field "level" in result: {addr=0x000055555555fef0,func=[async] main::$_0::operator()(unsigned long) const [clone .destroy],file=<file>,line=80,arch=i386:x86-64}

I'm considering an upstream patch to GDB to allow producing levels for these synthetic frames. But in the short term, there is a simple fix we can do to handle them here: allow frames without a level.

The way the proposed renumbering works in DebuggedThread is that frames without a level get a sequential level, whereas frames with a level keep their real level. This has the effect of causing async frames to display the locals of the real frame below them. This is a necessary workaround for another GDB FrameFilter limitation - we must pass a real frame number to -stack-list-variables with --frame. So GDB does not support querying back locals for synthetic frames either. I'm working on a fix for that also (either by a patch to MIEngine to allow querying from a script, or a VSCode extension, or another GDB patch).

But for now, this fix is a small step that at least makes it possible to debug with a synthetic-frame producing FrameFilter and see a backtrace without crashing.