[libc++] Diagnose unused variables of container types (#203084)

This adds `[[gnu::warn_unused]]` to all container types currently
implemented. In the future new containers should be added to the list,
as well as other value types in the library.

Clang has `-Wunused`, which is a suite of warnings about unused
variables. However, that doesn't warn for a lot of class types,
especially non-trivial ones. That is done to avoid false-positives for
RAII-style types like `lock_guard`. `[[gnu::warn_unused]]` exists to
tell Clang that a given class is a value type.

GitOrigin-RevId: ab8b037ef7b398aa864959a69f328fa69b567f74