Implement LWG 2193. Default constructors for standard library containers are explicit. Note that libc++ already did this for string/deque/forward_list/list/vector and the unordered containers; implement it for set/multiset/map/multimap. Add tests for all the containers. Two drive-by fixes as well: add a missing explicit in <deque>, and remove a tab that snuck into a container test. This issue is also LLVM bug 15724, and resolves it.