Change InstantiateType to be an Action (#2660)

Added an alternative way of Instantiating types (it was previously done using `InstantiateType` method).
Created an `Action` called `TypeInstantiationAction` and changed most of the code that directly invokes `InstantiateType` to instead spawn a new `TypeInstantiationAction`.

Relates to [#2594 ](https://github.com/carbon-language/carbon-lang/issues/2594)

explorer/interpreter/action_stack.cpp

@@ -130,6 +130,7 @@ static auto FinishActionKindFor(Action::Kind kind) -> FinishActionKind {
     case Action::Kind::ExpressionAction:
     case Action::Kind::WitnessAction:
     case Action::Kind::LValAction:
+    case Action::Kind::TypeInstantiationAction:
       return FinishActionKind::Value;
     case Action::Kind::StatementAction:
     case Action::Kind::DeclarationAction: