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.h

@@ -9,6 +9,7 @@
 
 #include "common/ostream.h"
 #include "executable_semantics/ast/expression.h"
+#include "executable_semantics/ast/pattern.h"
 #include "executable_semantics/ast/statement.h"
 #include "executable_semantics/interpreter/stack.h"
 #include "executable_semantics/interpreter/value.h"
@@ -19,6 +20,7 @@ namespace Carbon {
 enum class ActionKind {
   LValAction,
   ExpressionAction,
+  PatternAction,
   StatementAction,
   ValAction,
 };
@@ -33,6 +35,11 @@ struct ExpressionAction {
   const Expression* exp;
 };
 
+struct PatternAction {
+  static constexpr ActionKind Kind = ActionKind::PatternAction;
+  const Pattern* pattern;
+};
+
 struct StatementAction {
   static constexpr ActionKind Kind = ActionKind::StatementAction;
   const Statement* stmt;
@@ -46,6 +53,7 @@ struct ValAction {
 struct Action {
   static auto MakeLValAction(const Expression* e) -> Action*;
   static auto MakeExpressionAction(const Expression* e) -> Action*;
+  static auto MakePatternAction(const Pattern* p) -> Action*;
   static auto MakeStatementAction(const Statement* s) -> Action*;
   static auto MakeValAction(const Value* v) -> Action*;
 
@@ -53,6 +61,7 @@ struct Action {
 
   auto GetLValAction() const -> const LValAction&;
   auto GetExpressionAction() const -> const ExpressionAction&;
+  auto GetPatternAction() const -> const PatternAction&;
   auto GetStatementAction() const -> const StatementAction&;
   auto GetValAction() const -> const ValAction&;
 
@@ -75,7 +84,9 @@ struct Action {
   std::vector<const Value*> results;
 
  private:
-  std::variant<LValAction, ExpressionAction, StatementAction, ValAction> value;
+  std::variant<LValAction, ExpressionAction, PatternAction, StatementAction,
+               ValAction>
+      value;
 };
 
 }  // namespace Carbon