Fix to allow entering cooperative GC mode when the thread is in a forbid-suspend...

Fix to allow entering cooperative GC mode when the thread is in a forbid-suspend-for-debugger region (#42587)

- Followup to https://github.com/dotnet/runtime/pull/40060
- In short timing windows if a thread is placed in a pending-suspend-for-debugger state while in a forbid-suspend-for-debugger region, from the above PR it would not suspend for the debugger but it was missed that it can also get stuck in a perpetual spin-wait while entering cooperative GC mode. This causes the thread to not suspend for the debugger (VS times out after waiting) and the thread can only progress by unmarking it for debugger suspension.
- Fixed to break the spin-wait by checking whether the thread is in the forbid region

Fix for https://github.com/dotnet/runtime/issues/42375 in master