Fix compare_exchange_* with only one memory_order

Previously, compare_exchange_weak and compare_exchange_strong with only one
memory_order were defined as being equivalent to passing that value as both
the success and failure arguments of the overload accepting two memory_orders.
This is a bug, because the overload accepting two memory_orders clearly states
that failure does not accept certain values of memory_order.

The definitions of the equivalent functions in ISO C++ do not have this issue,
and so this commit aligns the behavior of these functions with ISO C++.

Author: Pennycook

adoc/chapters/programming_interface.adoc

@@ -20587,7 +20587,20 @@ bool compare_exchange_weak(T& expected, T desired,
                            memory_order order = default_read_modify_write_order,
                            memory_scope scope = default_scope) const
 ----
-   a@ Equivalent to [code]#compare_exchange_weak(expected, desired, order, order, scope)#.
+   a@ Equivalent to:
+[source, c++]
+----
+memory_order success = order;
+memory_order failure;
+if (order == memory_order::acq_rel) {
+  failure = memory_order::acquire;
+} else if (order == memory_order::release) {
+  failure = memory_order::relaxed;
+} else {
+  failure = order;
+}
+return compare_exchange_weak(expected, desired, success, failure, scope);
+----
 
 a@
 [source]
@@ -20617,7 +20630,20 @@ bool compare_exchange_strong(
     T& expected, T desired,
     memory_order order = default_read_modify_write_order) const
 ----
-   a@ Equivalent to [code]#compare_exchange_strong(expected, desired, order, order, scope)#.
+   a@ Equivalent to:
+[source, c++]
+----
+memory_order success = order;
+memory_order failure;
+if (order == memory_order::acq_rel) {
+  failure = memory_order::acquire;
+} else if (order == memory_order::release) {
+  failure = memory_order::relaxed;
+} else {
+  failure = order;
+}
+return compare_exchange_strong(expected, desired, success, failure, scope);
+----
 
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