Core: Assert that only the Host thread may call PauseAndLock().

[AdmiralCurtiss]

Core's PauseAndLock() states that // WARNING: PauseAndLock is not fully threadsafe so is only valid on the Host Thread, but there's absolutely no protection against doing this.

This adds an ASSERT() that we are indeed the host thread. To do that, this moves DolphinQt's IsHostThread() function into Core, and adds relevant DeclareAsHostThread() calls to all entry points.

This may expose erroneous usages of Core::RunAsCPUThread() and similar functions, which if found should be fixed before this is merged.

[iwubcode]

This is a pretty neat idea. LGTM

[rpaulucci3]

Hi. Sorry if this is not the correct place to post this, but the assertion seems to introduce an error right after connecting a real Wii Remote on Windows. This happens in build 5.0-19548, as shown in the screenshot.

[AdmiralCurtiss]

Yeah I'm not surprised to see a few of those pop up, we'll fix them as they get reported or run into them ourselves.

More repro notes: No game has to be running, Continous Scanning has to be enabled. The Wiimote Scanning thread is quite explicitly calling RunAsCPUThread, that's definitely evil.

dolphin/Source/Core/Core/HW/WiimoteReal/WiimoteReal.cpp

Lines 948 to 951 in bab3229

	Core::RunAsCPUThread([i, &wm] {
	g_wiimotes[i] = std::move(wm);
	WiimoteCommon::UpdateSource(i);
	});

[rpaulucci3]

I'm not at all familiar with the Dolphin codebase, but if this is a relatively simple fix, I'd be happy to try my hand at helping address it whenever possible :)