The value of dynamic_extent was orginally -1 when the extent was signed,
and now that the extent is unsigned it has become the maximum value
(~0U).

As the value of the extent is encoded in the mangled symbol names, the
current value of dynamic_extent generates symbol names like (using
clear() as an example)

   Itanium : _ZNKSt4spanIiLm4294967295EE5clearEv
   MSVC : ?clear@?$span_at_H$0PPPPPPPP@@std@@QEBAXXZ

In other words, the default extent generates long symbol names.

I propose that zero would be a better value for dynamic_extent. With
this value the above-mentioned symbols becomes:

   Itanium : _ZNKSt4spanIiLm0EE5clearEv
   MSVC : ?clear@?$span_at_H$0A@@std@@QEBAXXZ

In the concrete examples, this is a reduction of 9 and 7 characters
respectively for each symbol.