Implement most of P0607: Inline Variables for the Standard Library. This involved marking a lot of variables as inline (but only for C++17 and later).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@321658 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -18,7 +18,7 @@ namespace std
 {
 
 struct allocator_arg_t { };
-constexpr allocator_arg_t allocator_arg = allocator_arg_t();
+inline constexpr allocator_arg_t allocator_arg = allocator_arg_t();
 
 template <class T, class Alloc> struct uses_allocator;
 
@@ -631,6 +631,9 @@ template <class T> struct hash;
 template <class T, class D> struct hash<unique_ptr<T, D> >;
 template <class T> struct hash<shared_ptr<T> >;
 
+template <class T, class Alloc>
+  inline constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;
+
 // Pointer safety
 enum class pointer_safety { relaxed, preferred, strict };
 void declare_reachable(void *p);