[libcxx-commits] [libcxx] [libc++] mark `std::expected` as `nodiscard` (PR #130820)

[Mark de Wever via libcxx-commits]

https://github.com/mordante requested changes to this pull request.

Thanks for working on this!

This needs tests to verify that the `nodiscard` works as expected. 
`libcxx/test/std/containers/associative/multimap/empty.verify.cpp`
contains an example test.


https://github.com/llvm/llvm-project/pull/130820