[libc++] Use forwarding refs for predicates and projections in algorithms helpers

[imdj]

Replace lvalue references with forwarding references for predicates and projections in algorithms helpers.

This point has been raised a couple of times in reviews:

• [libc++] Forward more algorithms to the classic algorithms #114674 (comment)
• [libc++] Optimizing is_permutation #129565 (comment)

Few helper functions already do this:

• llvm-project/libcxx/include/__algorithm/is_permutation.h

  Lines 73 to 80 in 96925fa

  	_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool __is_permutation_impl(
  	_Iter1 __first1,
  	_Sent1 __last1,
  	_Iter2 __first2,
  	_Sent2 __last2,
  	_Pred&& __pred,
  	_Proj1&& __proj1,
  	_Proj2&& __proj2) {

• llvm-project/libcxx/include/__algorithm/ranges_partition.h

  Line 47 in 96925fa

  __partition_fn_impl(_Iter&& __first, _Sent&& __last, _Pred&& __pred, _Proj&& __proj) {

[ldionne]

Or even take by value and then pass around std::reference_wrapper. We should evaluate the pros/cons of each.