Fix MLIR compilation bugs for NVPTX target (#177024)

Changes:
- Change `ptxCode.c_str()` to `ptxCode.str().c_str()` to avoid error:
`error: 'class llvm::StringRef' has no member named 'c_str'; did you
mean 'str'?`
- Change `std::nullopt;` to `return mlir::failure();` to avoid error:
`could not convert 'std::nullopt' from 'const std::nullopt_t' to
'llvm::FailureOr<llvm::SmallVector<char, 0> >'`

Extra info:
- Tested versions: tried`llvmorg-21.1.8`, `llvmorg-22.1.0-rc1`,
`llvmorg-23-init`, `main`, all cannot compile without these fixes
- Test environment: `nvidia/cuda:13.1.0-devel-ubuntu24.04` docker image
(comes with gcc 13.3.0 and nvcc 13.1)
- Compile command: just turn-on
`-DLLVM_TARGETS_TO_BUILD="Native;NVPTX"`,
`-DMLIR_ENABLE_NVPTXCOMPILER=ON` and you will see the bugs.

A full command for example:
```
cmake llvm \
    -B build \
    -G Ninja \
    -DLLVM_ENABLE_PROJECTS=mlir \
    -DCMAKE_BUILD_TYPE=Release \
    -DMLIR_ENABLE_BINDINGS_PYTHON=ON \
    -DPython3_EXECUTABLE=python3 \
    -DLLVM_ENABLE_RTTI=ON \
    -DLLVM_INSTALL_UTILS=ON \
    -DLLVM_INCLUDE_TESTS=OFF \
    -DMLIR_INCLUDE_TESTS=OFF \
    -DLLVM_BUILD_EXAMPLES=OFF \
    -DLLVM_ENABLE_ASSERTIONS=ON \
    -DLLVM_CCACHE_BUILD=ON \
    -DCUDACXX=nvcc \
    -DCUDA_PATH=/usr/local/cuda \
    -DCMAKE_CUDA_ARCHITECTURES="80;89;120" \
    -DCMAKE_C_COMPILER=gcc \
    -DCMAKE_CXX_COMPILER=g++ \
    -DCMAKE_CUDA_COMPILER=nvcc \
    -DLLVM_TARGETS_TO_BUILD="Native;NVPTX" \
    -DMLIR_ENABLE_CUDA_RUNNER=ON \
    -DMLIR_ENABLE_CUDA_CONVERSIONS=ON \
    -DMLIR_ENABLE_NVPTXCOMPILER=ON
cmake --build build -j$(nproc) -t install
```

---------

Co-authored-by: Mehdi Amini <joker.eph@gmail.com>

0 files changed, 0 insertions, 0 deletions