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

[Alsey Coleman Miller via cfe-commits]

colemancda wrote:

I am not part of the Swift compiler team and I don't believe there is any documentation for that. ARM64 and x86_64 do have that documentation.

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