Do not wait in ClearReferent

Make IsNullOrMarkedHeapReference use a CAS when the GC requires an
update, to make it safe to run it concurrently with clearing the
referent. This means we no longer have to wait in ClearReferent.

Bug: 399794032
Test: Treehugger
Change-Id: I599980bb093f674d83591b0c5dd9066d09a589af

art

diff --git a/art b/art
index d7b8a30..ea6f741 160000
--- a/art
+++ b/art
@@ -1 +1 @@
-Subproject commit d7b8a30c974d62e6a84e8dee17383ef030bcd6bc
+Subproject commit ea6f741e73dce964994c88f4c85a1be7ecbe44c0