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

[Wang Pengcheng via cfe-commits]

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@@ -2415,7 +2415,10 @@ only be a power of 2 between 64 and 65536.
 For types where LMUL!=1, ``__riscv_v_fixed_vlen`` needs to be scaled by the LMUL
 of the type before passing to the attribute.
 
-``vbool*_t`` types are not supported at this time.
+For ``vbool*_t`` types, ``__riscv_v_fixed_vlen`` needs to be divided by the
+number from the type name. For example, ``vbool8_t`` needs to use
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wangpc-pp wrote:

> ``__riscv_v_fixed_vlen`` needs to be divided by the number from the type name.

Can this be done by compiler?

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