[basic.types] Replace macro constant with constexpr variable

Author: Kojoley

source/basic.tex

@@ -4481,7 +4481,7 @@
 subsequently hold its original value.
 \begin{example}
 \begin{codeblock}
-#define N sizeof(T)
+constexpr std::size_t N = sizeof(T);
 char buf[N];
 T obj;                          // \tcode{obj} initialized to its original value
 std::memcpy(buf, &obj, N);      // between these two calls to \tcode{std::memcpy}, \tcode{obj} might be modified