[expr.unary.op] Add a note about dereferencing a dangling pointer (cplusplus#8130)

Dereferencing a dangling pointer to an object is valid; the resulting
lvalue can be used in limited ways as per [basic.life] p8.

Add a non-normative note linking the two sections.

Author: dangelog

source/expressions.tex

@@ -5238,6 +5238,9 @@
 the result denotes that object or function;
 otherwise, the behavior is undefined except as specified in \ref{expr.typeid}.
 \begin{note}
+Indirection through a pointer to an out-of-lifetime object is valid\iref{basic.life}.
+\end{note}
+\begin{note}
 \indextext{type!incomplete}%
 Indirection through a pointer to an incomplete type (other than
 \cv{} \keyword{void}) is valid. The lvalue thus obtained can be