Revert r344529 "Implement the first part of the calendar support for C++20"

Revert r344535 "Wrap up the new chrono literals in an #ifdef..."
Revert r344546 "Mark a couple of test cases as 'C++17-only'..."

Some of the buildbot failures were masked by another error,
and this one was probably missed.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@344580 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/variant/variant.variant/variant.status/valueless_by_exception.pass.cpp

@@ -34,13 +34,11 @@
 #include "variant_test_helpers.hpp"
 
 int main() {
-#if TEST_STD_VER == 17
-  { // This test does not pass on C++20 or later; see https://bugs.llvm.org/show_bug.cgi?id=39232
+  {
     using V = std::variant<int, ConstexprTestTypes::NoCtors>;
     constexpr V v;
     static_assert(!v.valueless_by_exception(), "");
   }
-#endif
   {
     using V = std::variant<int, long, std::string>;
     const V v("abc");