Check and lowering support for for loops. (#5698)

Add check support for `for` loops following #1885. This also adds a
basic `Optional` type to the prelude, as that's necessary to support the
new `Iterate` interface.

Depends on #5688, #5697. Those PRs aren't stacked here, but this change
will crash until they land.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/node_stack.h

@@ -419,10 +419,10 @@ class NodeStack {
       case Parse::NodeKind::WhereOperand:
         return Id::KindFor<SemIR::InstId>();
       case Parse::NodeKind::ExplicitParamList:
+      case Parse::NodeKind::ForIn:
       case Parse::NodeKind::IfCondition:
       case Parse::NodeKind::IfExprIf:
       case Parse::NodeKind::ImplicitParamList:
-      case Parse::NodeKind::WhileCondition:
       case Parse::NodeKind::WhileConditionStart:
         return Id::KindFor<SemIR::InstBlockId>();
       case Parse::NodeKind::FunctionDefinitionStart:
@@ -449,6 +449,7 @@ class NodeStack {
       case Parse::NodeKind::ExplicitParamListStart:
       case Parse::NodeKind::FieldInitializer:
       case Parse::NodeKind::FieldIntroducer:
+      case Parse::NodeKind::ForHeaderStart:
       case Parse::NodeKind::FunctionIntroducer:
       case Parse::NodeKind::IfStatementElse:
       case Parse::NodeKind::ImplicitParamListStart:
@@ -483,10 +484,8 @@ class NodeStack {
       case Parse::NodeKind::ExportIntroducer:
       case Parse::NodeKind::FileEnd:
       case Parse::NodeKind::FileStart:
-      case Parse::NodeKind::Forall:
       case Parse::NodeKind::ForHeader:
-      case Parse::NodeKind::ForHeaderStart:
-      case Parse::NodeKind::ForIn:
+      case Parse::NodeKind::Forall:
       case Parse::NodeKind::IdentifierNameQualifierWithParams:
       case Parse::NodeKind::IdentifierNameQualifierWithoutParams:
       case Parse::NodeKind::IdentifierPackageName:
@@ -528,6 +527,7 @@ class NodeStack {
       case Parse::NodeKind::StructFieldDesignator:
       case Parse::NodeKind::StructTypeLiteralComma:
       case Parse::NodeKind::TupleLiteralComma:
+      case Parse::NodeKind::WhileCondition:
         return Id::Kind::Invalid;
       default:
         // In this case, the kind must be determinable from the category, or we