[RISCV] Backport c92ad411f2f94d8521cd18abcb37285f9a390ecb to release/18.x

[topperc]

The review #76551 was not approved in time for the branch, but I'd like to see __riscv_v_fixed_vlen for vbool support in clang 18.

Initial commit had to be reverted due to a clang IRGen changed that happened while it was sitting in review. So the cherry-picked commit here is a Recommit.

[topperc]

/cherry-pick c92ad41

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: Craig Topper (topperc)

The review #76551 was not approved in time for the branch, but I'd like to see __riscv_v_fixed_vlen for vbool support in clang 18.

Initial commit had to be reverted due to a clang IRGen changed that happened while it was sitting in review. So the cherry-picked commit here is a Recommit.

[github-actions]

/branch llvmbot/llvm-project/issue79479

[nikic]

Manually opened PR at #79596.

[topperc]

/branch topperc/llvm-project/pr/vbool-18.x

[nikic]

@topperc This is no longer supported, you should directly open a PR from your branch.