Unbork `std::memory_order` ABI.

Summary:
We need to pin the underlying type of C++20' `std::memory_order` to match the C++17 version. Anything less is an ABI break.

At the moment it's `unsigned` before C++20 and `int` after. Or if you're using `-fshort-enums` it's `unsigned char` before C++20 and `int` after.

This patch explicitly specifies the underlying type of the  C++20 `memory_order` to be w/e type the compiler would have chosen for the C++17 version.

Reviewers: mclow.lists, ldionne

Reviewed By: ldionne

Subscribers: jfb, jdoerfert, #libc, zoecarver

Differential Revision: https://reviews.llvm.org/D59063

llvm-svn: 355755

libcxx/include/atomic

@@ -584,10 +584,29 @@ void atomic_signal_fence(memory_order m) noexcept;
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+// Figure out what the underlying type for `memory_order` would be if it were
+// declared as an unscoped enum (accounting for -fshort-enums). Use this result
+// to pin the underlying type in C++20.
+enum __legacy_memory_order {
+    __mo_relaxed,
+    __mo_consume,
+    __mo_acquire,
+    __mo_release,
+    __mo_acq_rel,
+    __mo_seq_cst
+};
+
+typedef underlying_type<__legacy_memory_order>::type __memory_order_underlying_t;
+
 #if _LIBCPP_STD_VER > 17
 
-enum class memory_order {
-  relaxed, consume, acquire, release, acq_rel, seq_cst
+enum class memory_order : __memory_order_underlying_t {
+  relaxed = __mo_relaxed,
+  consume = __mo_consume,
+  acquire = __mo_acquire,
+  release = __mo_release,
+  acq_rel = __mo_acq_rel,
+  seq_cst = __mo_seq_cst
 };
 
 inline constexpr auto memory_order_relaxed = memory_order::relaxed;
@@ -600,14 +619,18 @@ inline constexpr auto memory_order_seq_cst = memory_order::seq_cst;
 #else
 
 typedef enum memory_order {
-  memory_order_relaxed, memory_order_consume, memory_order_acquire,
-  memory_order_release, memory_order_acq_rel, memory_order_seq_cst
+  memory_order_relaxed = __mo_relaxed,
+  memory_order_consume = __mo_consume,
+  memory_order_acquire = __mo_acquire,
+  memory_order_release = __mo_release,
+  memory_order_acq_rel = __mo_acq_rel,
+  memory_order_seq_cst = __mo_seq_cst,
 } memory_order;
 
 #endif // _LIBCPP_STD_VER > 17
 
-typedef underlying_type<memory_order>::type __memory_order_underlying_t;
-
+static_assert(is_same<underlying_type<memory_order>::type, __memory_order_underlying_t>::value,
+  "unexpected underlying type for std::memory_order");
 
 #if defined(_LIBCPP_HAS_GCC_ATOMIC_IMP) || \
 	defined(_LIBCPP_ATOMIC_ONLY_USE_BUILTINS)