libstdc++: Reuse predicates in std::search and std::is_permutation

libstdc++: Reuse predicates in std::search and std::is_permutation

Hoist construction of the call wrappers out of the loop when we're
repeatedly creating a call wrapper with the same bound arguments.

We need to be careful about iterators that return proxy references,
because bind1st(pred, *first) could bind a reference to a prvalue proxy
reference returned by *first. That would then be an invalid reference by
the time we invoked the call wrapper.

If we dereference the iterator first and store the result of that on the
stack, then we don't have a prvalue proxy reference, and can bind it (or
the value it refers to) into the call wrapper:

  auto&& val = *first; // lifetime extension
  auto wrapper = bind1st(pred, val);
  for (;;)
    /* use wrapper */;

This ensures that the reference returned from *first outlives the call
wrapper, whether it's a proxy reference or not.

For C++98 compatibility in __search we can use __decltype(expr) instead
of auto&&.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algobase.h (__search, __is_permutation):
	Reuse predicate instead of creating a new one each time.
	* include/bits/stl_algo.h (__is_permutation): Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>