[libcxx] [llvm] [clang-tools-extra] [libc++] Implement LWG3940: std::expected<void, E>::value() also needs E to be copy constructible (PR #71819)

[via cfe-commits]

PragmaTwice wrote:

https://github.com/llvm/llvm-project/actions/runs/7404652886/job/20147039107?pr=71819
```
# .---command stderr------------
# | error: 'expected-error' diagnostics expected but not seen: 
# |   File * Line * (directive at /home/runner/_work/llvm-project/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/value.lwg3940.verify.cpp:46): call to deleted constructor of 'MoveOnly'
# |   File * Line * (directive at /home/runner/_work/llvm-project/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/value.lwg3940.verify.cpp:47): call to deleted constructor of 'CopyOnly'
# |   File * Line * (directive at /home/runner/_work/llvm-project/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/value.lwg3940.verify.cpp:48): call to deleted constructor of 'CopyOnly'
# | 3 errors generated.
# `-----------------------------
```

It seems in some specific version/config of clang, these errors do not appear.

May I ask if there is any method to workaround these non-important error messages?

https://github.com/llvm/llvm-project/pull/71819