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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202994 91177308-0d34-0410-b5e6-96231b3b80d8

include/deque

@@ -1208,7 +1208,7 @@ public:
     deque()
         _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value)
         {}
-    _LIBCPP_INLINE_VISIBILITY deque(const allocator_type& __a) : __base(__a) {}
+    _LIBCPP_INLINE_VISIBILITY explicit deque(const allocator_type& __a) : __base(__a) {}
     explicit deque(size_type __n);
 #if _LIBCPP_STD_VER > 11
     explicit deque(size_type __n, const _Allocator& __a);