[libcxx][nfc] Cleanup libc++ specific tests.

[Mordante]

Summary

Move the tests to libcxx so they no longer need REQUIRES: libc++.
Verify tests don't need REQUIRES: libc++.

libcxx/test/libcxx/input.output/filesystems/class.path/path.member/path.native.obs/string_alloc.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
-// REQUIRES: libc++
 
 // These tests require locale for non-char paths
 // UNSUPPORTED: libcpp-has-no-localization
 
 // <filesystem>
 
 // class path
 

libcxx/test/libcxx/utilities/memory/util.smartptr/util.smartptr.shared/libcxx.control_block_layout.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
-// REQUIRES: libc++
 
 // This test makes sure that the control block implementation used for non-array
 // types in std::make_shared and std::allocate_shared is ABI compatbile with the
 // original implementation.
 //
 // This test is relevant because the implementation of that control block is
 // different starting in C++20, a change that was required to implement P0674.
 

libcxx/test/std/iterators/iterator.requirements/iterator.cust/iterator.cust.move/iter_move.nodiscard.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: gcc-10
 
 // Test the [[nodiscard]] extension in libc++.
-// REQUIRES: libc++
 
 // template<class I>
 // unspecified iter_move;
 
 #include <iterator>
 
 struct WithADL {
   WithADL() = default;