[clang] [C23] Add INFINITY and NAN macros to <float.h> (PR #96659)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

I'm not sure about tying this to __FINITE_MATH_ONLY__; -ffinite-math-only doesn't mean infinity doesn't exist, it just means you're promising that you won't use floating-point arithmetic/comparison ops on infinity.  Which is weird, but that's fast-math.  Also, other implementations don't do this.

In C99, INFINITY and NAN are macros in math.h, not float.h.  Is it correct to check `__STDC_VERSION__ >= 199901L`?

https://github.com/llvm/llvm-project/pull/96659