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

Author: ian-twilightcoder

clang/lib/Headers/float.h

@@ -18,21 +18,12 @@
  * additional definitions provided for Windows.
  * For more details see http://msdn.microsoft.com/en-us/library/y0ybw9fy.aspx
  *
- * Also fall back on Darwin and AIX to allow additional definitions and
+ * Also fall back on AIX to allow additional definitions and
  * implementation-defined values.
  */
-#if (defined(__APPLE__) || defined(__MINGW32__) || defined(_MSC_VER) ||        \
-     defined(_AIX)) &&                                                         \
+#if (defined(__MINGW32__) || defined(_MSC_VER) || defined(_AIX)) &&            \
     __STDC_HOSTED__ && __has_include_next(<float.h>)
 
-/* Prior to Apple's 10.7 SDK, float.h SDK header used to apply an extra level
- * of #include_next<float.h> to keep Metrowerks compilers happy. Avoid this
- * extra indirection.
- */
-#ifdef __APPLE__
-#define _FLOAT_H_
-#endif
-
 #  include_next <float.h>
 
 /* Undefine anything that we'll be redefining below. */