[clang] [llvm] [RISCV] Inline Assembly Support for GPR Pairs ('Pr') (PR #112983)

[Sam Elliott via cfe-commits]

lenary wrote:

Kito and I have agreed `R` is a good way forwards for GPR pairs, and the c-api-doc PR is approved (but not landed). I'll reword the description/message to update our constraint choice when I land this, which I hope to get to later today. 

https://github.com/llvm/llvm-project/pull/112983