Add pcl::utils::ignore function to remove Doxygen warnings for unused arguments

[aPonza]

As I was discussing in #3932 (comment) adding PCL_UNUSED as it was was causing -Wunused-values in GCC, Clang and XCode. This solves the issue at least for the first two, so if XCode is ok with it I'm too.

[aPonza]

OOM on windows

[kunaltyagi]

Should I expect a follow up PR touching all the (void) coming next?

If it's not too much bother, please create an issue anyways.

[taketwo]

Sorry, I'm late for the party (just a bit 😆). Did you consider backporting std::ignore that is available in C++17?

[aPonza]

I could really only find references saying not to use it because it's not its intended use (e.g. SO). Moreover from its syntax I wouldn't really bet on it working that well with Doxygen. Can you point me to a "do use it" source?

Also it seems to be some inline constexpr implementation in GCC which is from C++1z (inline variables in general are the new bit): link. Clang uses still an inline constexpr but with an unsigned char(?): link.

[taketwo]

Can you point me to a "do use it" source?

I don't have any such source; only the logic that it's better to use standard solutions when possible. But actually, I was not aware that ignore has an affinity with tuples. I use it to silence "unused variable" warnings everywhere.

[kunaltyagi]

I create a global lambda static constexpr auto ignore = [](...) {}; and then ignore(var_a, var_b, var_c);