[PATCH] PR libstdc++/87135 Rehash only when necessary (LWG2156)

[François Dumont]

All changes limited to hashtable_c++0x.cc file.

_Prime_rehash_policy::_M_next_bkt now really does what its comment was 
declaring that is to say:
 Â  // Return a prime no smaller than n.

_Prime_rehash_policy::_M_need_rehash rehash only when _M_next_size is 
exceeded, not only when it is reach.

 Â Â Â  PR libstdc++/87135
 Â Â Â  * src/c++11/hashtable_c++0x.cc:
 Â Â Â  (_Prime_rehash_policy::_M_next_bkt): Return a prime no smaller than
 Â Â Â  requested size, but not necessarily greater.
 Â Â Â  (_Prime_rehash_policy::_M_need_rehash): Rehash only if target size is
 Â Â Â  strictly greater than next resize threshold.
 Â Â Â  * testsuite/23_containers/unordered_map/modifiers/reserve.cc: Adapt 
test
 Â Â Â  to validate that there is no rehash as long as number of insertion is
 Â Â Â  lower or equal to the reserved number of elements.

unordered_map tests successful, ok to commit once all other tests 
completed ?

François
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