Use the correct alignment for POD-member memcpys where the first field is a
bitfield. CGBitField::StorageAlignment holds the alignment in chars, but
emitMemcpy had been treating it as if it were held in bits, leading to
underaligned memcpys.

Related to PR15348.

Thanks very much to Chandler for the diagnosis.
Don't cache the visibility of types.

Since r175326 an implicitly hidden template argument can cause a template
installation to become hidden, even if the template itself has an explicit
default visibility. This requires that we keep track of "late" additions
of the visibility attribute.

This is hopefully the last followup change. It just removes the caching of
visibilities from types so that we can see new attributes even after a type has
been used.