[clang] [CIR][RISCV][NFC] Add CIRGenBuiltinRISCV file to support RISCV builtins codegen (PR #186050)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> I think technically this is not a NFC?

Very technically. Any of these builtins would issue a "not yet implemented" diagnostic somewhere, so this doesn't change any useable functionality, and there would be no benefit in adding a test.

https://github.com/llvm/llvm-project/pull/186050