mention the extra const UB

Author: RalfJung

src/behavior-considered-undefined.md

@@ -60,6 +60,10 @@ code.
     > `rustc_layout_scalar_valid_range_*` attributes.
 * Incorrect use of inline assembly. For more details, refer to the [rules] to
   follow when writing code that uses inline assembly.
+* **In `const` context** (i.e., during the evaluation of a `const`/`static`
+  initializer, array length, enum discriminant, or const generic value):
+  transmuting or otherwise reinterpreting a pointer into some allocated object
+  as an integer.
 
 **Note:** Uninitialized memory is also implicitly invalid for any type that has
 a restricted set of valid values. In other words, the only cases in which