[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/test/std/strings/basic.string/string.cons/iter_alloc_deduction.fail.cpp

@@ -28,7 +28,7 @@
 
 #include "test_macros.h"
 
-class NotAnItertor {};
+class NotAnIterator {};
 
 template <typename T>
 struct NotAnAllocator { typedef T value_type; };
@@ -36,7 +36,7 @@ struct NotAnAllocator { typedef T value_type; };
 int main(int, char**)
 {
     { // Not an iterator at all
-    std::basic_string s1{NotAnItertor{}, NotAnItertor{}, std::allocator<char>{}}; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'basic_string'}}
+    std::basic_string s1{NotAnIterator{}, NotAnIterator{}, std::allocator<char>{}}; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'basic_string'}}
     }
     { // Not an input iterator
     std::basic_string<char16_t> s0;