Add BYN_WARN_UNUSED macro and apply to Name and IString

[sbc100]

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.

[kripken]

This feels odd to me, to scatter these throughout the codebase. Is there no global flag we can set?

[sbc100]

I mean its kind of like llvm/llvm-project#203084.

If there was a flag I guess it would be something like "-falways-assume-ctors-have-no-side-effects" but I'm pretty sure that is never always true so you wouldn't want it to be global.

[kripken]

Oh, I see, so this is for the ctors? Can we annotate the ctors directly, then, not the entire class?

[sbc100]

It seems like the way to do this is to annotate the class itself.

Annotating a C++ class with [[gnu::warn_unused]] tells the compiler to issue a -Wunused-variable warning if a variable of that type is instantiated but never explicitly used.

[kripken]

Is this documented somewhere? I can't seem to find GNU docs.

[sbc100]

The clang docs describe it: https://clang.llvm.org/docs/AttributeReference.html#warn-unused

"""
This attribute is available in both C and C++ language modes but is primarily useful in C++ for classes which have a non-trivial constructor or destructor but act as a value type rather than an RAII type.
"""

[kripken]

Thanks... funny that it is a gnu thing but clang documents it 😄

[sbc100]

Funnily enough I couldn't fine any other classes in binaryen what had latent unused variables like the ones in this PR.