In C23 mode, both float.h and math.h are required to define INIFINITY and NAN. However, with clang modules, there needs to be a single owner for the declarations. Let that be float.h since that's the suggested true owner in Annex F.2.2 of ISO/IEC 9899:2024, and introduce `__need_infinity_nan` so that math.h can pick up the compiler definitions.