basis_universal: Sync with latest Git to solve more warnings

[akien-mga]

Follow-up to:

• Scons: Remove system includes #111346

I PR'ed some additional warning fixes upstream, included here:

• Workaround Clang -Wnontrivial-memcall, clean up some unnecessary GCC -Wclass-memaccess pragmas BinomialLLC/basis_universal#404

[akien-mga]

FYI @Repiteo we didn't need that change. This is a .cpp file so it's compiled in env_thirdparty with disable_warnings() so it shouldn't raise warnings for us.

The case where we get warnings from third-party code is when they're emitted by headers which we include in first-party Godot code compiled with warnings enabled. That's what -isystem etc. aimed to work around, by marking third-party headers as "system" so they don't generate internal warnings.

Anyway I proposed a fix for that one upstream too:

• Use C++17 [[fallthrough]] to make intentional fallthroughs explicit BinomialLLC/basis_universal#405

But I'm not including it in this PR as we don't need it, and we want to minimize the amount of patches we have to maintain downstream.

[Repiteo]

Shoot, so it is! Must've forgot to limit my regex search to headers

[Repiteo]

Looks good!

[Repiteo]

Thanks!