Silence some Win64 clang-cl warnings about unused stuff due to ifdefs

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230685 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/CommandLineTest.cpp

@@ -35,6 +35,8 @@ class TempEnvVar {
 #if HAVE_SETENV
     // Assume setenv and unsetenv come together.
     unsetenv(name);
+#else
+    (void)name; // Suppress -Wunused-private-field.
 #endif
   }