All primitive types are @Interned

Fixes typetools#6371.

The interning checker does not apply `@Interned` to every occurrence of
every primitive type.  This allows some language constructs to
fabricate non-interned primitive values, leading to false positives.

For instance, in the new `loopVariables` test, the loop variable `i`
was assigned the type `@UnknownInterned long`.  Thus `i` could not be
used as an argument to `addExact` which requires interned arguments.

checker/src/main/java/org/checkerframework/checker/interning/InterningAnnotatedTypeFactory.java

@@ -242,6 +242,13 @@ public Void visitDeclared(AnnotatedDeclaredType t, Void p) {
       }
       return super.visitDeclared(t, p);
     }
+
+    @Override
+    public Void visitPrimitive(AnnotatedPrimitiveType t, Void p) {
+      // case 4: primitive types are interned
+      t.replaceAnnotation(INTERNED);
+      return super.visitPrimitive(t, p);
+    }
   }
 
   /**

checker/tests/interning/UnboxUninterned.java

@@ -10,4 +10,10 @@ void negation() {
     int i1 = -x.intValue();
     int i2 = -x;
   }
+
+  void loopVariables(java.util.List<Long> list) {
+    for (long i : list) {
+      long unused = Math.addExact(i, 0L);
+    }
+  }
 }