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

[Sam Elliott via cfe-commits]

lenary wrote:

I won't be merging this today. Instead, two changes/updates:
- Moving to use `R` instead of `Pr`, because kito pointed out that GCC cannot use `P` for anything except immediate values (done as a fixup for the moment, so we can back it out if we're struggling to agree on letters)
- Removing a change that's being done in #116159 

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