chore(deps): update ext/llvm-project digest to c51a758 (riscv#1548)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [ext/llvm-project](https://redirect.github.com/llvm/llvm-project.git)
([changelog](https://redirect.github.com/llvm/llvm-project.git/compare/974b76899953e8f1b3f070c5ce8a3bbbbd29e813..c51a758d7fe4d2168e62dfca5e1ef66a20d60d65))
| digest | `974b768` → `c51a758` |

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