C++20 comments

[BenjaminTrapani]

Summary

Move constructors and assignment operators closer to top of commented decl

Clean up type names

Remove C11 atomic impl for std::atomic

Add error assertions

[BenjaminTrapani]

Rebase master

[BenjaminTrapani]

Further rebases

[ldionne]

Is this meant to be reviewed, or is this part of the atomic_ref patch? If the former, please abandon to clean up the review queue.

libcxx/test/std/atomics/atomics.types.generic/assign_to_atomic_ref_deleted.fail.cpp

 #include <atomic>
 #include <new>
 #include <cassert>
 
 int main(int, char**) {
   int val = 1;
   std::atomic_ref<int> t0(val);
   std::atomic_ref<int> t1(val);
-  t0 = t1;
+  t0 =
+      t1; // expected-error {{object of type 'std::atomic_ref<int>' cannot be assigned because its copy assignment operator is implicitly deleted}}
 
   return 0;
 }

libcxx/test/std/atomics/atomics.types.generic/trivially_copyable_ref.fail.cpp

 #include <chrono> // for nanoseconds
 
 struct NotTriviallyCopyable {
   NotTriviallyCopyable(int i) : i_(i) {}
   NotTriviallyCopyable(const NotTriviallyCopyable& rhs) : i_(rhs.i_) {}
   int i_;
 };
 
-template <class T, class>
+template <class T>
 void test(T t) {
-  std::atomic_ref<T> t0(t);
+  std::atomic_ref<T> t0(
+      t); // expected-error@atomic:* {{static_assert failed due to requirement 'is_trivially_copyable<NotTriviallyCopyable>::value' "std::atomic_ref<_Tp> requires that '_Tp' be a trivially copyable type"}}
 }
 
 int main(int, char**) {
   test(NotTriviallyCopyable(42));
 
   return 0;
 }