Fallthroughs should be explicit

[r-barnes]

The -Wimplicit-fallthrough flag identifies implicit fallthroughs and can make them into errors.

This is desirable because implicit fallthroughs have a high error rate.

For instance, this line looks like it should have a break before it.

To make fallthroughs explicit __attribute__((fallthrough)) can be added to the code. Adding a XNNPACK_FALLTHROUGH macro can make this compiler/language version dependent via preprocessor flags, if necessary.

[fbarchard]

The instance you point to I dont think is a bug... the handling is shared?
XNNPack will sometimes do fall thru for efficiency in kernels too, but I'll look at each and see if a fix is needed, thanks.
The first warning I got on an XNNPack build is in pytorch/cpuinfo and I think a legit bug, so I sent a PR for that
pytorch/cpuinfo#225