[clang] Make sure C99/C11 features in <float.h> are provided in C++11

Summary:
Previously, those #defines were only provided in C or when GNU extensions were
enabled. We need those #defines in C++11 and above, too.

Reviewers: jfb, eli.friedman

Subscribers: jkorous, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58149

llvm-svn: 353970