[llvm] [mlir] [mlir][ods] Add documentation on how to use sharded op definitions (NFC) (PR #89664)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

@Mogball , This PR broke our local build. 

We append `-DLOCAL_NAME` into `LLVM_TABLEGEN_FLANGS` in `llvm/lib/cmake/llvm/TableGen.cmake` as
```
list(APPEND LLVM_TABLEGEN_FLAGS "-DLOCAL_NAME")
```

And in `./llvm/lib/Target/PowerPC/PPC.td`, we check it for some downstream code as:
```
...
#ifdef LOCAL_NAME
...
#endif
```

Now we got error message from `mlir-src-sharder` as
```
mlir-src-sharder -op-shard-index=1 -DLOCAL_NAME llvm-project/mlir/test/lib/Dialect/Test/TestOps.cpp --write-if-changed -o tools/mlir/test/lib/Dialect/Test/TestOps.1.cpp -d tools/mlir/test/lib/Dialect/Test/TestOps.1.cpp.d
mlir-src-sharder: Unknown command line argument '-DLOCAL_NAME'.  Try: 'llvm-project/build/bin/mlir-src-sharder --help'
mlir-src-sharder: Did you mean '-I'?
```

https://github.com/llvm/llvm-project/pull/89664