Avoid reserved names in C++ code: "__Consuming" (#26720)

Double-underscored names are reserved for the C++ "implementation"
(language and standard library). Even though "__Consuming" isn't
likely to be part of the C++ standard any time soon, we should follow
the rules.

Note that the API digester will continue to use the string
"__Consuming" for now, even though the underscores aren't really
significant, to avoid invalidating existing dumps.

No functionality change.

include/swift/Serialization/ModuleFormat.h

@@ -300,7 +300,7 @@ using MetatypeRepresentationField = BCFixed<2>;
 enum class SelfAccessKind : uint8_t {
   NonMutating = 0,
   Mutating,
-  __Consuming,
+  Consuming,
 };
 using SelfAccessKindField = BCFixed<2>;