[mlir][python] Fix PyObjectRef copy/move assignment for MSVC (#186758)
PyObjectRef has a user-declared move constructor but no explicit
copy/move assignment operators. On at least some version of MSVC,
instantiation of operator= is forced, causing a compile error:
```
In file included from mlir/lib/Bindings/Python/Globals.cpp:9:
In file included from mlir/include/mlir/Bindings/Python/IRCore.h:16:
<MSVC>/include/vector(1461,27): error: object of type 'value_type' (aka 'mlir::python::mlir::PyDiagnostic::DiagnosticInfo') cannot be assigned because its copy assignment operator is implicitly deleted
1461 | *_Mid = *_First;
| ^
<MSVC>/include/vector(1539,9): note: in instantiation of function template specialization 'std::vector<mlir::python::mlir::PyDiagnostic::DiagnosticInfo>::_Assign_counted_range<mlir::python::mlir::PyDiagnostic::DiagnosticInfo *>' requested here
1539 | _Assign_counted_range(_Right_data._Myfirst, static_cast<size_type>(_Right_data._Mylast - _Right_data._Myfirst));
| ^
mlir/include/mlir/Bindings/Python/IRCore.h(1317,33): note: in instantiation of member function 'std::vector<mlir::python::mlir::PyDiagnostic::DiagnosticInfo>::operator=' requested here
1317 | struct MLIR_PYTHON_API_EXPORTED MLIRError {
| ^
mlir/include/mlir/Bindings/Python/IRCore.h(369,16): note: copy assignment operator of 'DiagnosticInfo' is implicitly deleted because field 'location' has a deleted copy assignment operator
369 | PyLocation location;
| ^
mlir/include/mlir/Bindings/Python/IRCore.h(304,45): note: copy assignment operator of 'PyLocation' is implicitly deleted because base class 'BaseContextObject' has a deleted copy assignment operator
304 | class MLIR_PYTHON_API_EXPORTED PyLocation : public BaseContextObject {
| ^
mlir/include/mlir/Bindings/Python/IRCore.h(300,20): note: copy assignment operator of 'BaseContextObject' is implicitly deleted because field 'contextRef' has a deleted copy assignment operator
300 | PyMlirContextRef contextRef;
| ^
mlir/include/mlir/Bindings/Python/IRCore.h(73,3): note: copy assignment operator is implicitly deleted because 'PyObjectRef<mlir::python::mlir::PyMlirContext>' has a user-declared move constructor
73 | PyObjectRef(PyObjectRef &&other) noexcept
| ^
1 error generated.
```
Encountered on this env:
```
Compiler: clang-cl 20.1.8
STL: MSVC 14.44.35207 (Visual Studio 2022 Enterprise)
Runner: windows-2022 (GitHub Actions)
```
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