Get rid of a bunch of 'unused variable' warnings in test when run with debug_level set. NFC

llvm-svn: 359672
diff --git a/libcxx/test/std/strings/basic.string/string.access/back.pass.cpp b/libcxx/test/std/strings/basic.string/string.access/back.pass.cpp
index dd2a59a..69b88cc 100644
--- a/libcxx/test/std/strings/basic.string/string.access/back.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.access/back.pass.cpp
@@ -52,7 +52,7 @@
 #ifdef _LIBCPP_DEBUG
     {
         std::string s;
-        char c = s.back();
+        (void) s.back();
         assert(false);
     }
 #endif