[clang] Fix broken include_next in float.h

Summary:
The code defines __FLOAT_H and then includes the next <float.h>, which is
guarded on __FLOAT_H so it gets skipped entirely. This commit uses the header
guard __CLANG_FLOAT_H, like other headers (such as limits.h) do.

Reviewers: jfb

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50276

llvm-svn: 339016