[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss of data", part 2/7.

These tests for some guy's transparent operator functors were needlessly truncating their
double results to int. Preserving the doubleness makes compilers happier. I'm following
existing practice by adding an "// exact in binary" comment when the result isn't a whole number.
(The changes from 6 to 6.0 and so forth are stylistic, not critical.)

Fixes D27539.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@289106 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/arithmetic.operations/negate.pass.cpp

@@ -34,7 +34,7 @@ int main()
     constexpr int foo = std::negate<int> () (3);
     static_assert ( foo == -3, "" );
 
-    constexpr int bar = std::negate<> () (3.0);
-    static_assert ( bar == -3, "" );
+    constexpr double bar = std::negate<> () (3.0);
+    static_assert ( bar == -3.0, "" );
 #endif
 }