libstdc++: Use named struct for __decay_copy

libstdc++: Use named struct for __decay_copy

In r12-1486-gcb326a6442f09cb36b05ce556fc91e10bfeb0cf6 I changed
__decay_copy to be a function object of unnamed class type. This causes
problems when importing the library headers:

error: conflicting global module declaration 'constexpr const std::ranges::__cust_access::<unnamed struct> std::ranges::__cust_access::__decay_copy'

The fix is to use a named struct instead of an anonymous one.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (__decay_copy): Name type.