[libc++][NFC] Refactor `[[nodiscard]]` tests (#173451)
Originally some `[[nodiscard]]` tests were implemented in
`*/test/libcxx/diagnostics`. The Standard has a library `Diagnostics`
and this folder should be reserved for it by convention. Most newer
tests were added to their respective sub-folders. This patch moves
around the already implemented `[[nodiscard]]` tests to their respective
folders where they belong and standardizes the name to
`nodiscard.verify.cpp` wherever possible.
N.B. This refactors only tests, which were merged. The remaining
(in-progress) ones will be moved in a future patch to reduce merge
conflicts.
