[Sema][SVE] Reject sizeless types in exception specs

In the current SVE ACLE spec, the usual rules for throwing and
catching incomplete types also apply to sizeless types.  However,
throwing pointers to sizeless types should not pose any real difficulty,
so as an extension, the clang implementation allows that.

This patch enforces these rules for explicit exception specs.

Differential Revision: https://reviews.llvm.org/D76087