[clang] [llvm] [clang][RISCV] Support the Swift calling convention (PR #213448)

[Sam Elliott via cfe-commits]

lenary wrote:

@rjmccall thanks for this comprehensive guide.

It would be good to know which of these end up relating to swift embedded (which I understand is a subset of swift). Part of the reason I ask is that Apple engineers have been working on RV32 support in Mach-O (upstream), which seems adjacent. From my perspective as a RISC-V backend reviewer/maintainer, I'm keener to review support that's reasonably holistic, rather than having separate efforts that don't meet in the middle - even if we document things as subject to future changes.

https://github.com/llvm/llvm-project/pull/213448