Adding MlirBool to C Bindings

Yeah, I’m not sure why we aren’t just using _Bool here, it seems portable enough and easy enough to #define for targets that don’t support it.

instead of:

inline int mlirAttributeIsNull(MlirAttribute attr) { return !attr.ptr; }

Just use:

inline _Bool mlirAttributeIsNull(MlirAttribute attr) { return !attr.ptr; }

Is there some disadvantage to that?

-Chris