[K/N][New MM] Disallow getting thread data for detached threads

This patch fixes a performance degradation introduced by
99bd26c2ef (Switch thread states
in termination handlers).

kotlin-native/runtime/src/main/cpp/Memory.h

@@ -389,11 +389,17 @@ namespace kotlin {
 namespace mm {
 
 // Returns the MemoryState for the current thread.
-// If the memory subsystem isn't initialized for the current thread, returns nullptr.
+// The current thread must be attached to the runtime.
 // Try not to use it very often, as (1) thread local access can be slow on some platforms,
 // (2) TLS gets deallocated before our thread destruction hooks run.
 MemoryState* GetMemoryState() noexcept;
 
+
+// TODO: Replace with direct access to ThreadRegistry when the legacy MM is gone.
+// Checks if the current thread is attached to the runtime.
+// This function accesses a TLS variable, so it must not be called from a thread destructor.
+bool IsCurrentThreadRegistered() noexcept;
+
 } // namespace mm
 
 enum class ThreadState {