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/static_scope.cpp

@@ -4,6 +4,8 @@
 
 #include "explorer/ast/static_scope.h"
 
+#include <optional>
+
 #include "explorer/common/error_builders.h"
 #include "llvm/Support/Error.h"
 
@@ -71,4 +73,30 @@ auto StaticScope::TryResolve(const std::string& name,
   return result;
 }
 
+auto StaticScope::AddReturnedVar(ValueNodeView returned_var_def_view)
+    -> ErrorOr<Success> {
+  std::optional<ValueNodeView> resolved_returned_var = ResolveReturned();
+  if (resolved_returned_var.has_value()) {
+    return CompilationError(returned_var_def_view.base().source_loc())
+           << "Duplicate definition of returned var also found at "
+           << resolved_returned_var->base().source_loc();
+  }
+  returned_var_def_view_ = std::move(returned_var_def_view);
+  return Success();
+}
+
+auto StaticScope::ResolveReturned() const -> std::optional<ValueNodeView> {
+  if (returned_var_def_view_.has_value()) {
+    return returned_var_def_view_;
+  }
+  for (Nonnull<const StaticScope*> parent : parent_scopes_) {
+    std::optional<ValueNodeView> parent_returned_var =
+        parent->ResolveReturned();
+    if (parent_returned_var.has_value()) {
+      return parent_returned_var;
+    }
+  }
+  return std::nullopt;
+}
+
 }  // namespace Carbon