Fix SVD for MSVC+CUDA.

CUDA gets confused about the definition of Index, and can't match the out-of-line definitions of a few functions with their declarations. If we just import the SVDBase::Index definition, then gcc/clang get confused about the out-of-line definition. To fix all, we import SVDBase::Index, and modify all definitions to use the internal Index type.

Also addressed annoying warnings about not returning anything at the end of a non-void function.