[SE-0371] Back-deploy support for main-actor-isolated deinit

When targeting a platform that predates the introduction of isolated
deinit, make a narrow exception that allows main-actor-isolated deinit
to work through a special, inlineable entrypoint that is
back-deployed. This implementation

  1. Calls into the real implementation when available, otherwise
  2. Checks if we're on the main thread, destroying immediately when
we are, otherwise
  3. Creates a new task on the main actor to handle destruction.

This implementation is less efficient than the implementation in the
runtime, but allows us to back-deploy this functionality as far back
as concurrency goes.

Fixes rdar://151029118.

lib/IRGen/IRGenModule.cpp

@@ -982,6 +982,14 @@ namespace RuntimeConstants {
     return RuntimeAvailability::AlwaysAvailable;
   }
 
+  RuntimeAvailability IsolatedDeinitAvailability(ASTContext &context) {
+    auto featureAvailability = context.getIsolatedDeinitAvailability();
+    if (!isDeploymentAvailabilityContainedIn(context, featureAvailability)) {
+      return RuntimeAvailability::ConditionallyAvailable;
+    }
+    return RuntimeAvailability::AlwaysAvailable;
+  }
+
   RuntimeAvailability
   MultiPayloadEnumTagSinglePayloadAvailability(ASTContext &context) {
     auto featureAvailability = context.getMultiPayloadEnumTagSinglePayload();