[C23] Support the SNAN macros in <float.h> (#162858)
These macros were brought in as part of the TS 18661-1 integration in
C23, and then renamed in WG14 N2710.
Test coverage is being added to WG14 N3364 from C2y because that paper
was fixing a bug with the way these macros are handled with unary + and
- operators, so all the existing test coverage was there, just needed to
use a header file instead of defining the macros manually.
Fixes #162830
