[clang] [clang][headers][Apple] Don't include_next float.h to avoid an unnecessary module dependency (PR #137432)

[John McCall via cfe-commits]

rjmccall wrote:

Messing around with `#include_next` is scary, but the problems are always thorny reasons that only come up in intense testing; I don't see anything obviously wrong, and I trust @ian-twilightcoder to have done the investigation properly. LGTM.

https://github.com/llvm/llvm-project/pull/137432