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

[John McCall via llvm-commits]

rjmccall wrote:

> But one thing that would be helpful for us is if we can find someone to be the maintainer for swift within the Clang community. I don't expect they'd have a lot of review work to do related to the role, but that gives contributors an idea of who to reach out for on PRs like this (or the 32-bit PPC swift calling convention PR that came up recently). Do you know of anyone with enough crossover between the projects who might be interested in stepping up for it?

I've had to step away from maintainership overall, but I'm happy to continue to serve in this role for Swift interactions.

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