[clang] [clang][RISCV] Support RVV bfloat16 C intrinsics (PR #89354)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

vfncvtbf16.c, vfwcvtbf16.c and vfwmaccbf16.c already in the LLVM repo, so I think those files could removed from this PR?

https://github.com/llvm/llvm-project/pull/89354