The switch to clang disabled the clang-specific -Wunused-value
since it generated some harmless warnings. Unfortunately, that also
prevent [[nodiscard]] violations from warning.
Fix by clearing all instances of the warning (including [[nodiscard]]
violations that crept in while it was disabled) and reinstating the warning.
Closes#7767
* github.com:scylladb/scylla:
build: reinstate -Wunused-value warning for [[nodiscard]]
test: lib: don't ignore future in compare_readers()
test: mutation_test: check both ranges when comparing summaries
serialializer: silence unused value warning in variant deserializer
c990f2bd34