[libc++] P0433R2: add the remaining deduction guides.

Add deduction guides to `valarray` and `scoped_allocator_adaptor`. This largely
finishes implementation of the paper:

* deduction guides for other classes mentioned in the paper were
  implemented previously (see the list below);
* deduction guides for several classes contained in the proposal
  (`reference_wrapper`, `lock_guard`, `scoped_lock`, `unique_lock`,
  `shared_lock`) were removed by [LWG2981](https://wg21.link/LWG2981).

Also add deduction guides to the synopsis for the few classes (e.g. `pair`)
where they were missing.

The only part of the paper that isn't fully implemented after this patch is
making sure certain deduction guides don't participate in overload resolution
when given incorrect template parameters.

List of significant commits implementing the other parts of P0433 (omitting some
minor fixes):

* [pair](af65856eec)
* [basic_string](6d9f750dec)
* [array](0ca8c0895c)
* [deque](dbb6f8a817)
* [forward_list](e076700b77)
* [list](4a227e582b)
* [vector](df8f754792)
* [queue/stack/priority_queue](5b8b8b5dce)
* [basic_regex](edd5e29cfe)
* [optional](f35b4bc395)
* [map/multimap](edfe8525de)
* [set/multiset](e20865c387)
* [unordered_set/unordered_multiset](296a80102a)
* [unordered_map/unordered_multimap](dfcd4384cb)
* [function](e1eabcdfad)
* [tuple](1308011e1b)
* [shared_ptr/weak_ptr](83564056d4)

Additional notes:
* It was revision 2 of the paper that was voted into the Standard.
  P0433R3 is a separate paper that is not part of the Standard.
* The paper also mandates removing several `make_*_searcher` functions
  (e.g. `make_boyer_moore_searcher`) which are currently not implemented
  (except in `experimental/`).
* The `__cpp_lib_deduction_guides` feature test macro from the paper was
  accidentally omitted from the Standard.

Differential Revision: https://reviews.llvm.org/D112510

libcxx/include/valarray

@@ -105,6 +105,8 @@ public:
     void resize(size_t n, value_type x = value_type());
 };
 
+template<class T, size_t cnt> valarray(const T(&)[cnt], size_t) -> valarray<T>;
+
 class slice
 {
 public:
@@ -1081,6 +1083,11 @@ private:
     valarray& __assign_range(const value_type* __f, const value_type* __l);
 };
 
+#if _LIBCPP_STD_VER > 14
+template<class _Tp, size_t _Size>
+valarray(const _Tp(&)[_Size], size_t) -> valarray<_Tp>;
+#endif
+
 _LIBCPP_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS void valarray<size_t>::resize(size_t, size_t))
 
 template <class _Op, class _Tp>