[NFC] AST: Add define for experimental coroutines.

Enable customizing the behavior of macro expansions of for accessors
which are experimental coroutines.

Author: nate-chandler

include/swift/AST/AccessorKinds.def

@@ -73,6 +73,14 @@
 #define COROUTINE_ACCESSOR(ID, KEYWORD) OPAQUE_ACCESSOR(ID, KEYWORD)
 #endif
 
+/// EXPERIMENTAL_COROUTINE_ACCESSOR(ID, KEYWORD, FEATURE)
+///   The given coroutine accessor is experimental.
+///
+///   Defaults to COROUTINE_ACCESSOR(ID, KEYWORD)
+#ifndef EXPERIMENTAL_COROUTINE_ACCESSOR
+#define EXPERIMENTAL_COROUTINE_ACCESSOR(ID, KEYWORD, FEATURE) COROUTINE_ACCESSOR(ID, KEYWORD)
+#endif
+
 /// ANY_ADDRESSOR(ID, KEYWORD)
 ///   The given keyword corresponds to an addressor of the given kind.
 ///
@@ -205,6 +213,7 @@ LAST_ACCESSOR(Init)
 #undef OBJC_ACCESSOR
 #undef OPAQUE_ACCESSOR
 #undef COROUTINE_ACCESSOR
+#undef EXPERIMENTAL_COROUTINE_ACCESSOR
 #undef OBSERVING_ACCESSOR
 #undef SINGLETON_ACCESSOR
 #undef ACCESSOR