[clang] [RISCV] Support __riscv_v_fixed_vlen for vbool types. (PR #76551)

[Wang Pengcheng via cfe-commits]

https://github.com/wangpc-pp approved this pull request.

Oh sorry, I was going to approve this before llvm 18 branch but I forgot.
Should we backport this to llvm 18?

https://github.com/llvm/llvm-project/pull/76551