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

[John McCall via llvm-commits]

rjmccall wrote:

Okay, I wrote up an [RFC](https://discourse.llvm.org/t/rfc-expectations-about-swiftcall-implementations/91560), and I have a [documentation PR open](https://github.com/llvm/llvm-project/pull/215915).

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