Misc shutdown fixes

* Destroy the ImGui debugger explicitly, not at static destruction time

* Windows: destroy the debugger windows before NativeShutdown()

* Clear the disassembly cache where it goes stale, not in view destructors
  (behind a no-op stub for libretro, which doesn't build DisassemblyManager.cpp)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Core/Debugger/DisassemblyManager.cpp

@@ -40,6 +40,10 @@
 
 DisassemblyManager g_disassemblyManager;
 
+void ClearDisassemblyCache() {
+	g_disassemblyManager.clear();
+}
+
 bool isInInterval(u32 start, u32 size, u32 value) {
 	return start <= value && value <= (start+size-1);
 }