Initial implementation of returned var. (#1348)

This PR implements the `returned var` for the explorer.
- Enabled `returned var` and `return var` syntax in lexer and parser.
- Split `Return` statement into `ReturnVar` and `ReturnExpression` to distinguish the two types of returns.
- Added logic in name resolution, type checking and interpretation to process `returned var` definition and `ReturnVar` statement.

explorer/interpreter/action_stack.cpp

@@ -131,7 +131,6 @@ auto ActionStack::FinishAction() -> ErrorOr<Success> {
     case Action::Kind::ExpressionAction:
     case Action::Kind::LValAction:
     case Action::Kind::PatternAction:
-      CARBON_FATAL() << "This kind of action must produce a result: " << *act;
     case Action::Kind::ScopeAction:
       CARBON_FATAL() << "ScopeAction at top of stack";
     case Action::Kind::StatementAction: