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/SILGen/SILGenFunction.cpp

@@ -141,6 +141,7 @@ DeclName SILGenModule::getMagicFunctionName(SILDeclRef ref) {
   case SILDeclRef::Kind::EnumElement:
     return getMagicFunctionName(cast<EnumElementDecl>(ref.getDecl())
                                   ->getDeclContext());
+  case SILDeclRef::Kind::AsyncEntryPoint:
   case SILDeclRef::Kind::EntryPoint:
     auto *file = ref.getDecl()->getDeclContext()->getParentSourceFile();
     return getMagicFunctionName(file);