Conjure up asyncMainDrainQueue

The asyncMainDrainQueue is also declared `internal`, so it won't show up
in the swiftinterface file.

The expected declaration is:

```
@available(SwiftStdlib 5.5, *)
@_silgen_name("swift_task_asyncMainDrainQueue")
internal func _asyncMainDrainQueue() -> Never
```

Author: etcwilde

lib/SILGen/SILGenFunction.cpp

@@ -959,6 +959,25 @@ void SILGenFunction::emitAsyncMainThreadStart(SILDeclRef entryPoint) {
 
   // Start Main loop!
   FuncDecl *drainQueueFuncDecl = SGM.getAsyncMainDrainQueue();
+  if (!drainQueueFuncDecl) {
+    // If it doesn't exist, we can conjure one up instead of crashing
+    // @available(SwiftStdlib 5.5, *)
+    // @_silgen_name("swift_task_asyncMainDrainQueue")
+    // internal func _asyncMainDrainQueue() -> Never
+    ParameterList *emptyParams = ParameterList::createEmpty(getASTContext());
+    drainQueueFuncDecl = FuncDecl::createImplicit(
+        getASTContext(), StaticSpellingKind::None,
+        DeclName(
+            getASTContext(),
+            DeclBaseName(getASTContext().getIdentifier("_asyncMainDrainQueue")),
+            /*Arguments*/ emptyParams),
+        {}, /*async*/ false, /*throws*/ false, {}, emptyParams,
+        getASTContext().getNeverType(),
+        entryPoint.getDecl()->getModuleContext());
+    drainQueueFuncDecl->getAttrs().add(new (getASTContext()) SILGenNameAttr(
+        "swift_task_asyncMainDrainQueue", /*implicit*/ true));
+  }
+
   SILFunction *drainQueueSILFunc = SGM.getFunction(
       SILDeclRef(drainQueueFuncDecl, SILDeclRef::Kind::Func), NotForDefinition);
   SILValue drainQueueFunc =