[Sema][SVE] Reject subscripts on pointers to sizeless types

clang currently accepts:

  __SVInt8_t &foo1(__SVInt8_t *x) { return *x; }
  __SVInt8_t &foo2(__SVInt8_t *x) { return x[1]; }

The first function is valid ACLE code and generates correct LLVM IR
(and assembly code).  But the second function is invalid for the
same reason that arrays of sizeless types are.  Trying to code-generate
the function leads to:

  llvm/include/llvm/Support/TypeSize.h:126: uint64_t llvm::TypeSize::getFixedSize() const: Assertion `!IsScalable && "Request for a fixed size on a s
calable object"' failed.

Another problem is that:

  template<typename T>
  constexpr __SIZE_TYPE__ f(T *x) { return &x[1] - x; }
  typedef int arr1[f((int *)0) - 1];
  typedef int arr2[f((__SVInt8_t *)0) - 1];

produces:

  a.cpp:2:48: warning: subtraction of pointers to type '__SVInt8_t' of zero size has undefined behavior [-Wpointer-arith]
  constexpr __SIZE_TYPE__ f(T *x) { return &x[1] - x; }
					   ~~~~~ ^ ~
  a.cpp:4:18: note: in instantiation of function template specialization 'f<__SVInt8_t>' requested here
  typedef int arr2[f((__SVInt8_t *)0) - 1];

This patch reports an appropriate diagnostic instead.

Differential Revision: https://reviews.llvm.org/D76084