Bug 90923 shows that even though GCC hash-table based containers
like hash_map can be instantiated on types with user-defined ctors
and dtors they invoke the dtors of such types without invoking
the corresponding ctors.

It was thanks to this bug that I spent a day debugging "interesting"
miscompilations during GCC bootstrap (in fairness, it was that and
bug 90904 about auto_vec copy assignment/construction also being
hosed even for POD types).

The attached patch corrects the hash_map and hash_set templates
to invoke the ctors of the elements they insert and makes them
(hopefully) safe to use with non-trivial user-defined types.

Tested on x86_64-linux.

Martin