Revert rGfae7b98c221b5b28797f7b56b656b6b819d99f27 "[Support] Change...

Revert rGfae7b98c221b5b28797f7b56b656b6b819d99f27 "[Support] Change SetVector's default template parameter to SmallVector<*, 0>"

This is failing on Windows MSVC builds:
llvm\unittests\Support\ThreadPool.cpp(380): error C2440: 'return': cannot convert from 'Vector' to 'std::vector<llvm::BitVector,std::allocator<llvm::BitVector>>'
        with
        [
            Vector=llvm::SmallVector<llvm::BitVector,0>
        ]