MFC r305392:

For kernel builds, instead of suppressing certain clang warnings, make
them non-fatal, so there is some incentive to fix them eventually.

MFC r333819 (by mmacy):

Silence non-actionable warnings in vendor code

We can't modify vendor code so there's no signal in warnings from it.
Similarly -Waddress-of-packed-member is not useful on networking code
as access to packed structures is fundamental to its operation.