Factor out a Pattern sum type from Expression (#685)

`Pattern` is intended to pilot some changes I would like to apply to all our sum types:
- The alternatives are expressed as derived classes rather than members of a `std::variant`.
- The alternatives are classes in the [style guide sense](https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes), meaning they can have invariants, but can't have public data members.
- Creating an object is expressed using a constructor rather than a factory function.
- Accessing an alternative is expressed as a cast (using LLVM's RTTI system) rather than `std::get` or a `Get` method.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/action.cpp

@@ -29,6 +29,12 @@ auto Action::MakeExpressionAction(const Expression* e) -> Action* {
   return act;
 }
 
+auto Action::MakePatternAction(const Pattern* p) -> Action* {
+  auto* act = new Action();
+  act->value = PatternAction({.pattern = p});
+  return act;
+}
+
 auto Action::MakeStatementAction(const Statement* s) -> Action* {
   auto* act = new Action();
   act->value = StatementAction({.stmt = s});
@@ -49,6 +55,10 @@ auto Action::GetExpressionAction() const -> const ExpressionAction& {
   return std::get<ExpressionAction>(value);
 }
 
+auto Action::GetPatternAction() const -> const PatternAction& {
+  return std::get<PatternAction>(value);
+}
+
 auto Action::GetStatementAction() const -> const StatementAction& {
   return std::get<StatementAction>(value);
 }
@@ -65,6 +75,9 @@ void Action::Print(llvm::raw_ostream& out) const {
     case ActionKind::ExpressionAction:
       out << *GetExpressionAction().exp;
       break;
+    case ActionKind::PatternAction:
+      out << *GetPatternAction().pattern;
+      break;
     case ActionKind::StatementAction:
       GetStatementAction().stmt->PrintDepth(1, out);
       break;