Placate unused variable warnings uncovered by improvements to clang's -Wunused-variable

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@315809 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/numerics/c.math/ctgmath.pass.cpp

@@ -14,6 +14,7 @@
 int main()
 {
     std::complex<double> cd;
+    (void)cd;
     double x = std::sin(0);
     ((void)x); // Prevent unused warning
 }