Add PushState overload to combine Parse::State change with StateStackEntry (#3485)

Shortens a common `PushState` pattern

toolchain/parse/handle_statement.cpp

@@ -120,9 +120,7 @@ auto HandleStatementForHeader(Context& context) -> void {
 
 auto HandleStatementForHeaderIn(Context& context) -> void {
   auto state = context.PopState();
-
-  state.state = State::StatementForHeaderFinish;
-  context.PushState(state);
+  context.PushState(state, State::StatementForHeaderFinish);
   context.PushState(State::Expr);
 }
 
@@ -151,9 +149,7 @@ auto HandleStatementIf(Context& context) -> void {
 
 auto HandleStatementIfConditionFinish(Context& context) -> void {
   auto state = context.PopState();
-
-  state.state = State::StatementIfThenBlockFinish;
-  context.PushState(state);
+  context.PushState(state, State::StatementIfThenBlockFinish);
   context.PushState(State::CodeBlock);
 }
 
@@ -162,8 +158,7 @@ auto HandleStatementIfThenBlockFinish(Context& context) -> void {
 
   if (context.ConsumeAndAddLeafNodeIf(Lex::TokenKind::Else,
                                       NodeKind::IfStatementElse)) {
-    state.state = State::StatementIfElseBlockFinish;
-    context.PushState(state);
+    context.PushState(state, State::StatementIfElseBlockFinish);
     // `else if` is permitted as a special case.
     context.PushState(context.PositionIs(Lex::TokenKind::If)
                           ? State::StatementIf
@@ -182,8 +177,7 @@ auto HandleStatementIfElseBlockFinish(Context& context) -> void {
 
 auto HandleStatementReturn(Context& context) -> void {
   auto state = context.PopState();
-  state.state = State::StatementReturnFinish;
-  context.PushState(state);
+  context.PushState(state, State::StatementReturnFinish);
 
   context.AddLeafNode(NodeKind::ReturnStatementStart, context.Consume());
 
@@ -227,8 +221,7 @@ auto HandleStatementWhile(Context& context) -> void {
 auto HandleStatementWhileConditionFinish(Context& context) -> void {
   auto state = context.PopState();
 
-  state.state = State::StatementWhileBlockFinish;
-  context.PushState(state);
+  context.PushState(state, State::StatementWhileBlockFinish);
   context.PushState(State::CodeBlock);
 }