Add AsyncEntryPoint SILDeclRef type

The AsyncEntryPoint represents the thunk that is wrapped in a task. This
thunk is used to ensure that the main function explicitly calls "exit",
and to properly unwrap and report any unhandled errors returned from the
user-written main. The function takes on the name `@async_main` in the
emitted SIL.

lib/IRGen/GenObjC.cpp

@@ -474,6 +474,7 @@ namespace {
       case SILDeclRef::Kind::PropertyWrapperBackingInitializer:
       case SILDeclRef::Kind::PropertyWrapperInitFromProjectedValue:
       case SILDeclRef::Kind::EntryPoint:
+      case SILDeclRef::Kind::AsyncEntryPoint:
         llvm_unreachable("Method does not have a selector");
 
       case SILDeclRef::Kind::Destroyer: