[libc++] Simplify the implementation of __tuple_leaf

[philnik777]

We can use __conditional_no_unique_address instead of specializating __tuple_leaf for empty non-final classes.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 51809e4a26a8c6db6cce115822d185fe662dc0fc f20f4b981ffb1bc438cefdb1ce8b17135d31aeb4 --extensions ,h,cpp -- libcxx/include/__utility/conditional_no_unique_address.h libcxx/include/__expected/expected.h libcxx/include/__utility/in_place.h libcxx/include/tuple libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp

View the diff from clang-format here.

diff --git a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp b/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp
index 0f61c6b1e1..b5f86ac829 100644
--- a/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp
+++ b/libcxx/test/libcxx/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.verify.cpp
@@ -38,7 +38,6 @@ template <class T> struct CannotDeduce {
 template <class ...Args>
 void F(typename CannotDeduce<std::tuple<Args...>>::type const&) {}
 
-
 void f() {
   // Test that we emit our diagnostic from the library.
   // expected-error@tuple:* 8 {{Attempted construction of reference element binds to a temporary whose lifetime has ended}}