[libc++] Add `explicit` to a bunch of internal detail ctors.

Notably the following ctors remain non-explicit because they
are used as implicit conversions in too many places:
* __debug_less(_Compare&)
* __map_iterator(_TreeIterator)
* __map_const_iterator(_TreeIterator)
* __hash_map_iterator(_HashIterator)
* __hash_map_const_iterator(_HashIterator)

Differential Revision: https://reviews.llvm.org/D119894