libstdc++: [_Hashtable] Consider all initializer_list elements are inserted

When instantiated using an initializer_list the container is pre-sized 
based on
initializer_list size.

libstdc++-v3/ChangeLog:

     * include/bits/hashtable_policy.h 
(_Insert_base<>::insert(initializer_list<>)):
     Use assignment operator if container is empty and has default 
bucket count.
     * include/bits/hashtable.h (_Hashtable<>(initializer_list<>)): Use 
initializer_list
     size as bucket count hint if user did not provide any value that is 
to say if it is
     the default 0 value.
     * testsuite/23_containers/unordered_set/init-list.cc (test02): New 
test case.

Tested under Linux x86_64.

François