[Sema][SVE] Don't allow static or thread-local variables to have sizeless type
clang accepts a TU containing just:
However, sizeless types are not allowed to have static or thread-local
storage duration and trying to code-generate the TU triggers an LLVM
Globals cannot contain scalable vectors <vscale x 16 x i8>* @x fatal error: error in backend: Broken module found, compilation aborted!
This patch adds an associated clang diagnostic.
Differential Revision: https://reviews.llvm.org/D75736