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

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

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D106673

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

@@ -7,7 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
-// REQUIRES: libc++
 
 // These tests require locale for non-char paths
 // UNSUPPORTED: libcpp-has-no-localization

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

@@ -7,7 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 // 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

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

@@ -11,7 +11,6 @@
 // UNSUPPORTED: gcc-10
 
 // Test the [[nodiscard]] extension in libc++.
-// REQUIRES: libc++
 
 // template<class I>
 // unspecified iter_move;