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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Is there discussion somewhere of why you need a command-line flag?  ABI-modifying flags tend to be dangerous to work with. It's hard to ensure that all code is compiled with the same flags, and if the user messes up, the compiler silently generates broken code.

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