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

This adopts a similar behavior to AArch64 SVE, where bool vectors are
represented as a vector of chars with 1/8 the number of elements. This
ensures the vector always occupies a power of 2 number of bytes.

A consequence of this is that vbool64_t, vbool32_t, and vool16_t can
only be used with a vector length that guarantees at least 8 bits.

clang/docs/ReleaseNotes.rst

@@ -1290,6 +1290,8 @@ PowerPC Support
 - Added support for ``builtin_cpu_supports`` on AIX, along with a subset of
   features that can be queried.
 
+- ``__attribute__((rvv_vector_bits(N))) is now supported for RVV vbool*_t types.
+
 CUDA/HIP Language Changes
 ^^^^^^^^^^^^^^^^^^^^^^^^^