This patch handles some (but not all) cases of types with a `packed`
attribute that contain a type with an `align(N)` attribute.

This uses information available in the Clang IR about types' layout to
determine if a given type is likely to have an alignment attribute
placed on it during the code generation phase. This is just a heuristic;
the decision to actually place an alignment attribute depends on
information that is not known until code generation, so the logic here
may result in both false positives and false negatives.

Using the real information from codegen on whether an alignment
attribute was placed on a child type is not possible in general, because
the order in which types are generated is not guaranteed. In some cases
code may be generated for parent types before code is generated for
child types contained in that parent. Then, it will be impossible to
make an accurate decision regarding whether to remove the `packed`
attribute from the parent type.

The impact of a false negative is that an outer type may have a `packed`
attr even when an inner type as an `align` attr. Such a type would not
compile under rustc, but it already would not compile so this has no
harmful impact.

The impact of a false positive is that an outer type may have its
`packed` attr stripped needlessly, because no inner types actually have
an `align` attr. Because we only remove the `packed` attr when we can
confirm that there will be no change to the type's layout, this also
should have no harmful impact.