Add BYN_WARN_UNUSED macro and apply to Name and IString (#8973)

Define `BYN_WARN_UNUSED` using `[[gnu::warn_unused]]` for GCC/Clang and
fallback for other compilers. Annotating `IString` and `Name` with this
attribute allows compilers to report unused variable diagnostics for
default-constructed instances regardless of non-trivial constructors.

Followup to #8972.

Once this lands we can extend this to other types to get better
coverage.