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

[Craig Topper via cfe-commits]

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@@ -137,6 +137,7 @@ set(riscv_files
   sifive_vector.h
   andes_vector.h
   riscv_mips.h
+  riscv_simd.h
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topperc wrote:

this list was supposed to be alphabetized, but it looks like that got lost on the last couple additions. Please put this after riscv_ntlh.

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