[Clang] Use C++17 in constant-builtins-fmax.cpp test

Add `-std=c++17` to the test so that buildbot won't fail

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D135486

clang/test/Sema/constant-builtins-fmax.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify %s
 // expected-no-diagnostics
 
 constexpr double NaN = __builtin_nan("");