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

[Sam Elliott via cfe-commits]

lenary wrote:

I believe we've had some attempts at swift support on RISC-V before. Broadly, we want docs about how Swift lowers to RISC-V, so we can actually review these changes against them. 

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