[clang][CodeGen] Switch declaration of vtable information to be [0 x ptr] (#65596)

Continuing the discussion in
https://discourse.llvm.org/t/codegen-layout-of-si-class-type-info-doesnt-match-the-actual-size/73274

Before we had this code:
@_ZTVN10__cxxabiv117__class_type_infoE = external global ptr

now we'll produce:
@_ZTVN10__cxxabiv117__class_type_infoE = external global [0 x ptr]

This is because we may not know the exact size of this data, and clang
issues gep inbounds with idx=2. Before, that gep would always result in
poison.