[libc++] [LIBCXX-DEBUG-FIXME] Stop using invalid iterators to insert into sets/maps.

This simply applies Howard's commit 4c80bfbd consistently
across all the associative and unordered container tests.

"unord.set/insert_hint_const_lvalue.pass.cpp" failed with `-D_LIBCPP_DEBUG=1`
before this patch; it was the only one that incorrectly reused
invalid iterator `e`. The others already used valid iterators
(generally `c.end()`); I'm just making them all match the same pattern
of usage: "e, then r, then c.end() for the rest."

Differential Revision: https://reviews.llvm.org/D101679