[clang] [llvm] [RISCV][VLS] Support RISCV VLS calling convention (PR #100346)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Is there a downside if we just recommend people `__attribute__((riscv_vls_cc(MY_LIBRARY_VLEN)))`, then put `-DMY_LIBRARY_VLEN=128` on the command-line?  It's about the same complexity as -mriscv-abi-vlen, and it's much more obvious that what you're doing is fragile.

https://github.com/llvm/llvm-project/pull/100346