Get tests running with warnings. Fix warnings in headers and tests

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@228344 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/date.time/tested_elsewhere.pass.cpp

@@ -21,6 +21,7 @@
 int main()
 {
     std::clock_t c = 0;
+    ((void)c); // avoid unused warning
     std::size_t s = 0;
     std::time_t t = 0;
     std::tm tm = {0};