[libcxx] [test] Remove a Clang/C2 workaround.

[STL_MSFT]

Summary

[libcxx] [test] Remove a Clang/C2 workaround.

Clang/LLVM doesn't need this workaround.

[CaseyCarter]

As the party who originally put this workaround in place, I'll happily certify that it never affected any platforms that anyone but we Microsofters care about. I'll take responsibility for any buildbot fallout - forward any nasty notices to me.

test/std/utilities/optional/optional.object/optional.object.mod/reset.pass.cpp

int main()
     {
         optional<X> opt;
         static_assert(noexcept(opt.reset()) == true, "");
         assert(X::dtor_called == false);
         opt.reset();
         assert(X::dtor_called == false);
         assert(static_cast<bool>(opt) == false);
     }
-    assert(X::dtor_called == false); // TRANSITION, Clang/C2 VSO#239997
     {
         optional<X> opt(X{});
         X::dtor_called = false;
         opt.reset();
         assert(X::dtor_called == true);
         assert(static_cast<bool>(opt) == false);
         X::dtor_called = false;
     }
-    assert(X::dtor_called == false); // TRANSITION, Clang/C2 VSO#239997
 }