Add tests for LWG#2299. While doing so, I noticed that the tests we have for...

Add tests for LWG#2299. While doing so, I noticed that the tests we have for the transparent comparators don't actually call them. Fix those tests, too. Now one of them is failing, due to a missing const in <map>. Add that (twice). Next step is to do the same for <unordered_map>

llvm-svn: 241091