Mark java.time.InstantSource and java.time.Clock as thread safe.

Also, annotate `FakeTimeSource` with `@ThreadSafe` to fix breakages caused by marking `InstantSource` as thread safe. `TimeSource` and `common.time.Clock` should also be annotated directly, but we would need to fix or suppress all existing implementations of them first, so I left a TODO instead.

Context: http://yaqs/6914049073339170816

Tested:
    TAP for global presubmit queue
    []
PiperOrigin-RevId: 705936781

core/src/main/java/com/google/errorprone/bugpatterns/threadsafety/WellKnownThreadSafety.java

@@ -63,8 +63,10 @@ private static ImmutableMap<String, AnnotationInfo> buildThreadSafeClasses(
     new MapBuilder()
         .addAll(mutability.getKnownImmutableClasses())
         .addStrings(extraKnownThreadSafe)
-        .add(ClassLoader.class)
-        .add(Thread.class)
+        .add(java.lang.ClassLoader.class)
+        .add(java.lang.Thread.class)
+        .add(java.time.InstantSource.class)
+        .add(java.time.Clock.class)
         .add(java.util.Random.class)
         .add(java.util.concurrent.atomic.AtomicBoolean.class)
         .add(java.util.concurrent.atomic.AtomicInteger.class)
@@ -108,8 +110,6 @@ private static ImmutableMap<String, AnnotationInfo> buildThreadSafeClasses(
         .add(java.security.SecureRandom.class)
         .add("com.google.common.base.Ticker")
         .add("com.google.common.testing.FakeTicker")
-        .add("com.google.common.time.Clock")
-        .add("com.google.common.time.TimeSource")
         .add("com.google.common.util.concurrent.AtomicLongMap", "K")
         .add("com.google.common.util.concurrent.CheckedFuture", "V", "X")
         .add("com.google.common.util.concurrent.ListeningExecutorService")