Remove -Wpacked false positive for non-pod types where the layout isn't directly changed

The packed attribute can still be useful in this case if the struct is
then placed inside another packed struct - the non-pod element type's
packed attribute declares that it's OK to misalign this element inside
the packed structure. (otherwise the non-pod element is not packed/its
alignment is preserved, as per D117616/27712337)

Fixes PR62353

Differential Revision: https://reviews.llvm.org/D149182