[libc++] Harden usage of static_assert against C++03

In C++03, we emulate static_assert with a macro, and we must parenthesize
multiple arguments.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373328 91177308-0d34-0410-b5e6-96231b3b80d8

include/cmath

@@ -644,8 +644,8 @@ _LIBCPP_CONSTEXPR _IntT __max_representable_int_for_float() _NOEXCEPT {
   static_assert(is_floating_point<_FloatT>::value, "must be a floating point type");
   static_assert(is_integral<_IntT>::value, "must be an integral type");
   static_assert(numeric_limits<_FloatT>::radix == 2, "FloatT has incorrect radix");
-  static_assert(_IsSame<_FloatT, float>::value || _IsSame<_FloatT, double>::value
-               || _IsSame<_FloatT,long double>::value, "unsupported floating point type");
+  static_assert((_IsSame<_FloatT, float>::value || _IsSame<_FloatT, double>::value
+                 || _IsSame<_FloatT,long double>::value), "unsupported floating point type");
   return _FloatBigger ? numeric_limits<_IntT>::max() :  (numeric_limits<_IntT>::max() >> _Bits << _Bits);
 }