try to avoid #[repr(packed)] when align is needed

Currently rustc forbids compound types from having both a `packed` and
`align` attribute.

When a source type has both attributes, this may mean it cannot be
represented with the current rustc. Often, though, one or both of these
attributes is redundant and can be safely removed from the generated
Rust code.

Previously, bindgen avoided placing the `align` attribute when it is
not needed. However, it would always place the `packed` attribute if the
source type has it, even when it is redundant because the source type is
"naturally packed".

With this change, bindgen avoids placing `packed` on a type if the
`packed` is redundant and the type needs an `align` attribute. If the
type does not have an "align" attribute, then bindgen will still place
`packed` so as to avoid changing existing working behavior.

This commit also takes out an extraneous `is_packed()` call from
`StructLayoutTracker::new()` since the value can be passed in from the
caller; this avoids duplicating work in some cases.