[IRGen] Preserve trailing padding for last array element
With optimization, constant array literals are emitted as
static class objects comprising a 32-byte header followed
by the data.
However, the trailing padding was omitted in this case,
which trips ASan warnings and can lead to garbage data after
the array being read in some cases.
This modifies the IRGen emission to include the final padding.
Resolves rdar://191110241
