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/ast/declaration.h

@@ -11,6 +11,7 @@
 #include "common/ostream.h"
 #include "executable_semantics/ast/function_definition.h"
 #include "executable_semantics/ast/member.h"
+#include "executable_semantics/ast/pattern.h"
 #include "executable_semantics/ast/struct_definition.h"
 #include "executable_semantics/interpreter/address.h"
 #include "executable_semantics/interpreter/dictionary.h"
@@ -58,8 +59,10 @@ struct ChoiceDeclaration {
 struct VariableDeclaration {
   static constexpr DeclarationKind Kind = DeclarationKind::VariableDeclaration;
   int source_location;
-  std::string name;
-  const Expression* type;
+  // TODO: split this into a non-optional name and a type, initialized by
+  // a constructor that takes a BindingPattern and handles errors like a
+  // missing name.
+  const BindingPattern* binding;
   const Expression* initializer;
 };
 
@@ -74,8 +77,8 @@ class Declaration {
       int line_num, std::string name,
       std::list<std::pair<std::string, const Expression*>> alternatives)
       -> const Declaration;
-  static auto MakeVariableDeclaration(int source_location, std::string name,
-                                      const Expression* type,
+  static auto MakeVariableDeclaration(int source_location,
+                                      const BindingPattern* binding,
                                       const Expression* initializer)
       -> const Declaration;