Cleanup how debug comparators are created in <algorithm>

Instead of having an `#if` block in every algorithm using a debug
comparator, this patch introduces the __comp_ref_type trait that
selects __debug_less in debug mode and _Comp& otherwise.

This patch should have no observable functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@358252 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed