…ssary module dependency (#137432)

float.h doesn't define anything in Apple's SDKs that the clang float.h
doesn't undefine, so all the include_next does is add an unnecessary
module dependency. Skip the include_next and completely shadow the SDK
header.

Backport of llvm/llvm-project@557ddc2

(cherry picked from commit 7e4318b)

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

Backport of llvm/llvm-project@0e3fdc2

(cherry picked from commit 52497d8)

…IFINITY and NAN with float.h (#164348)

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.

Backport of llvm/llvm-project@38473c5

(cherry picked from commit ae49e19)

(cherry picked from commit 765d976)

It was introduced in commit c28b205 during the upgrade to LLVM 18
to fix the build on (basically) all platforms, and this remains true
on Linux. For Apple however, the newest MacOSX27.0.sdk is incomaptible
and the option must not be set.

(cherry picked from commit a314567)

Similar to ctype.h, this is also caused by the large std module that
wraps all submodules. Instead of removing, it seems sufficient to
mark the header as textual.

(cherry picked from commit 0f5bfad)