Enable building and using atomic shared_ptr for GCC.

Summary:
Currently the  implementation of [util.smartptr.shared.atomic] is provided only when using Clang, and not with GCC. This is a relic of not having a GCC implementation of <atomic>, even though <atomic> isn't actually used in the implementation. This patch enables support for atomic shared_ptr functions when using GCC.

Note that this is not a header only change. Previously only Clang builds of libc++.so would provide the required symbols. There is no reason  for this restriction.
After this change both Clang and GCC builds should be binary compatible with each other WRT these symbols.


Reviewers: mclow.lists, rmaprath, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21407

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@273076 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -5464,9 +5464,8 @@ inline _LIBCPP_INLINE_VISIBILITY
 basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os, shared_ptr<_Yp> const& __p);
 
-// TODO(EricWF): Enable this for both Clang and GCC. Currently it is only
-// enabled with clang.
-#if defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+
+#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 class _LIBCPP_TYPE_VIS __sp_mut
 {
@@ -5595,7 +5594,7 @@ atomic_compare_exchange_weak_explicit(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v
     return atomic_compare_exchange_weak(__p, __v, __w);
 }
 
-#endif  // defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+#endif  // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 //enum class
 struct _LIBCPP_TYPE_VIS pointer_safety

src/memory.cpp

@@ -124,7 +124,7 @@ __shared_weak_count::__get_deleter(const type_info&) const _NOEXCEPT
 
 #endif  // _LIBCPP_NO_RTTI
 
-#if defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 static const std::size_t __sp_mut_count = 16;
 static __libcpp_mutex_t mut_back_imp[__sp_mut_count] =
@@ -177,7 +177,7 @@ __get_sp_mut(const void* p)
     return muts[hash<const void*>()(p) & (__sp_mut_count-1)];
 }
 
-#endif // defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+#endif // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 void
 declare_reachable(void*)