Remove unnecessary non-const accessors in hash table bases

Remove unnecessary non-const accessors in hash table bases

The const accessors are OK (and arguably more correct) for most callers
to use. The _M_swap functions that use the non-const overloads can just
directly use the _S_get members of the EBO helpers.

	* include/bits/hashtable_policy.h (_Hash_code_base::_M_swap): Use
	_S_get accessors for members in EBO helpers.
	(_Hash_code_base::_M_extract(), _Hash_code_base::_M_ranged_hash())
	(_Hash_code_base::_M_h1(), _Hash_code_base::_M_h2()): Remove non-const
	overloads.
	(_Hashtable_base::_M_swap): Use _S_get accessors for members in EBO
	helpers.
	(_Hashtable_base::_M_eq()): Remove non-const overload.

From-SVN: r271286