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

[via cfe-commits]

sihuan wrote:

The motivation for naming it `riscv_simd.h` was to follow the existing experimental GCC implementation referenced in the [riscv-p-spec README](https://github.com/riscv/riscv-p-spec)
> v0.18 gcc:https://github.com/ruyisdk/riscv-gcc/tree/p-dev

I  agree with the concerns raised. `riscv_simd.h` is easy to confuse with `riscv_vector.h`. I think 'packed' fits this extension better. I am happy to rename it. Based on the discussion, `riscv_packed.h` seems to be the preferred name.

Does `riscv_packed.h` work for everyone? Since this patch only has packed intrinsics, should I just rename this file to `riscv_packed.h`?

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