Add exclusion for Reference.clear and reference processing

Prevents race conditions like unclearing cleared references or
calling IsMarkedHeapReference on null references.

Bug: 33389022

Test: test-art-host

Change-Id: Iee83b76d84453e929172f1a83f284aa4910e126c
diff --git a/runtime/native/java_lang_ref_Reference.cc b/runtime/native/java_lang_ref_Reference.cc
index bedca10..3f4573d 100644
--- a/runtime/native/java_lang_ref_Reference.cc
+++ b/runtime/native/java_lang_ref_Reference.cc
@@ -33,8 +33,15 @@
   return soa.AddLocalReference<jobject>(referent);
 }
 
+static void Reference_clear(JNIEnv* env, jobject javaThis) {
+  ScopedFastNativeObjectAccess soa(env);
+  ObjPtr<mirror::Reference> ref = soa.Decode<mirror::Reference>(javaThis);
+  Runtime::Current()->GetHeap()->GetReferenceProcessor()->ClearReferent(ref);
+}
+
 static JNINativeMethod gMethods[] = {
   NATIVE_METHOD(Reference, getReferent, "!()Ljava/lang/Object;"),
+  NATIVE_METHOD(Reference, clear, "!()V"),
 };
 
 void register_java_lang_ref_Reference(JNIEnv* env) {