[Sema][SVE] Reject aligned/_Alignas for sizeless types

A previous patch rejected alignof for sizeless types.  This patch
extends that to cover the "aligned" attribute and _Alignas.  Since
sizeless types are not meant to be used for long-term data, cannot
be used in aggregates, and cannot have static storage duration,
there shouldn't be any need to fiddle with their alignment.

Like with alignof, this is a conservative position that can be
relaxed in future if it turns out to be too restrictive.

Differential Revision: https://reviews.llvm.org/D75573