[Clang][RISCV] Expose vlenb to user

This commit adds function `vlenb` into riscv_vector.h. `vlenb` is defined
through builtin function `__builtin_rvv_vlenb`, which is lowered to
`llvm.read_register`.

Reviewed By: kito-cheng, pcwang-thead

Differential Revision: https://reviews.llvm.org/D141032