[libc++][NFC] Add a static assertion to document an assumption in std::hash (#114440)

The implementation of std::hash for unsigned long makes the (correct)
assumption that size_t is at least as large as unsigned long. If that
were not the case on a platform, the implementation of std::hash for
unsigned long would be absolutely terrible. Add a static assertion to
document that assumption.

Author: ldionne

libcxx/include/__functional/hash.h

@@ -406,7 +406,11 @@ struct _LIBCPP_TEMPLATE_VIS hash<long> : public __unary_function<long, size_t> {
 
 template <>
 struct _LIBCPP_TEMPLATE_VIS hash<unsigned long> : public __unary_function<unsigned long, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(unsigned long __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
+  _LIBCPP_HIDE_FROM_ABI size_t operator()(unsigned long __v) const _NOEXCEPT {
+    static_assert(sizeof(size_t) >= sizeof(unsigned long),
+                  "This would be a terrible hash function on a platform where size_t is smaller than unsigned long");
+    return static_cast<size_t>(__v);
+  }
 };
 
 template <>