[mdspan.layout.stride.expo] Convert to integral type to perform math

[mdspan.layout.stride.cons] uses either a span or array of a type which we can only convert to `index_type` as the second argument to the exposition-only `REQUIRED-SPAN-SIZE`. We must perform that conversion before doing math with the result.

Author: CaseyCarter

source/containers.tex

@@ -20419,8 +20419,12 @@
 \item
 otherwise \tcode{0}, if the size of the multidimensional index space \tcode{e} is 0,
 \item
-otherwise \tcode{1} plus the sum of products of \tcode{(e.extent($r$) - 1)} and \tcode{strides[$r$]}
-for all $r$ in the range $[0, \tcode{e.rank()})$.
+otherwise \tcode{1} plus the sum of products of
+\tcode{(e.extent($r$) - 1)} and
+\begin{codeblock}
+extents_type::@\exposid{index-cast}@(strides[@$r$@])
+\end{codeblock}
+  for all $r$ in the range $[0, \tcode{e.rank()})$.
 \end{itemize}
 
 \pnum