[clang] Add riscv_simd.h for P extension intrinsics (PR #181115)

[via cfe-commits]

sihuan wrote:

In the second commit, I updated the ABI handling to allow passing P-extension vectors directly (using `getDirect()`) instead of coercing them to integers.

Since the backend already supports code generation for generic vector types like <4 x i8> in GPRs, I think using native vector types in the frontend IR is cleaner and eliminates the need for bitcast in the intrinsics/tests.

Please let me know if this approach aligns with the current ABI guidelines for experimental extensions.

https://github.com/llvm/llvm-project/pull/181115