[libc++] Fix an exception safety issue in `forward_list` and add tests.

When inserting nodes into a forward list, each new node is allocated but
not constructed. The constructor was being called explicitly on the node
`value_` but the `next_` pointer remained uninitialized rather than
being set to null. This bug is only triggered in the cleanup code if an
exception is thrown -- upon successful creation of new nodes, the last
incorrect "next" value is overwritten to a correct pointer.

This issue was found due to new tests added in
https://reviews.llvm.org/D149830.

Differential Revision: https://reviews.llvm.org/D152327