libstdc++: Fix non-default constructors for hash containers [PR101583]
When I added the new mixin to _Hashtable, I forgot to explicitly
construct it in each non-default constructor. That means you can't
use any constructors unless all three of the hash function, equality
function, and allocator are all default constructible.
libstdc++-v3/ChangeLog:
PR libstdc++/101583
* include/bits/hashtable.h (_Hashtable): Replace mixin with
_Enable_default_ctor. Construct it explicitly in all
non-forwarding, non-defaulted constructors.
* testsuite/23_containers/unordered_map/cons/default.cc: Check
non-default constructors can be used.
* testsuite/23_containers/unordered_set/cons/default.cc:
Likewise.