[clang] [RISCV] Pass structs containing riscv_rvv_vector_bits types using vector registers. (PR #203601)

[Pengcheng Wang via cfe-commits]

https://github.com/wangpc-pp approved this pull request.

The code looks great to me. Should we document this in psabi or toolchain-convention?

https://github.com/llvm/llvm-project/pull/203601