incorrect setting of LLVM_ALLOW_PROBLEMATIC_CONFIGURATIONS option

Hi guys,
this line of `option` is incorrect https://github.com/llvm/llvm-project/blob/b23ba298af84d56be149350bf617dc869c1fb42c/llvm/cmake/modules/CheckProblematicConfigurations.cmake#L2
should be
```
option(LLVM_ALLOW_PROBLEMATIC_CONFIGURATIONS "Set this option to ON to allow problematic toolchain configurations. Use on your own risk." OFF)
```

Open llvm/cmake/modules/CheckProblematicConfigurations.cmake at line 2 and compare the LLVM_ALLOW_PROBLEMATIC_CONFIGURATIONS option declaration with the issue's proposed form. Check the surrounding CMake logic to confirm the option name, description, and default are used consistently, then run the relevant CMake configuration check. Done means the declaration matches the requested behavior without breaking configuration.

Written by the indexing model from the issue text.