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

[Brandon Wu via llvm-commits]

4vtomat 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.

Yeah, it's described in the [PR](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/418/files#diff-3a934f00cffdb3e509722753126a2cf6082a7648ab3b9ca8cbb0e84f8a6a12edR464-R466), basically this option won't affect functions that don't have `riscv_vls_cc` keyword specified, and I believe it provides a way for user to write `ABI_VLEN` agnostic code, but yeah it's easier to break the code if we don't handle carefully.

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