IRGen: Generate spare bit mask lazily in StructLayoutBuilder
Wait until we know the full size of the SpareBitVector before we
construct it from the spare bit masks of the individual elements
in the struct. This process mirrors the way we construct the rest
of the struct type.
This should reduce the number of short-lived allocations required
to build small struct types (especially after #25240). It also
provides a convenient place to insert code to handle the
construction of big-endian bit masks on when targeting big-endian
machines, assuming we decide to go down that route.
