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/SIL/IR/SILPrinter.cpp

@@ -316,6 +316,7 @@ void SILDeclRef::print(raw_ostream &OS) const {
   switch (kind) {
   case SILDeclRef::Kind::Func:
   case SILDeclRef::Kind::EntryPoint:
+  case SILDeclRef::Kind::AsyncEntryPoint:
     break;
   case SILDeclRef::Kind::Allocator:
     OS << "!allocator";