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/ast/statement.cpp

@@ -50,6 +50,9 @@ void Statement::PrintDepth(int depth, llvm::raw_ostream& out) const {
       break;
     case StatementKind::VariableDefinition: {
       const auto& var = cast<VariableDefinition>(*this);
+      if (var.is_returned()) {
+        out << "returned ";
+      }
       out << "var " << var.pattern() << " = " << var.init() << ";";
       break;
     }
@@ -71,8 +74,12 @@ void Statement::PrintDepth(int depth, llvm::raw_ostream& out) const {
       }
       break;
     }
-    case StatementKind::Return: {
-      const auto& ret = cast<Return>(*this);
+    case StatementKind::ReturnVar: {
+      out << "return var;";
+      break;
+    }
+    case StatementKind::ReturnExpression: {
+      const auto& ret = cast<ReturnExpression>(*this);
       if (ret.is_omitted_expression()) {
         out << "return;";
       } else {