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/member.cpp

@@ -6,11 +6,11 @@
 
 namespace Carbon {
 
-auto Member::MakeFieldMember(int line_num, std::string name,
-                             const Expression* type) -> Member* {
+auto Member::MakeFieldMember(int line_num, const BindingPattern* binding)
+    -> Member* {
   auto m = new Member();
   m->line_num = line_num;
-  m->value = FieldMember({.name = std::move(name), .type = type});
+  m->value = FieldMember({.binding = binding});
   return m;
 }
 
@@ -22,7 +22,7 @@ void Member::Print(llvm::raw_ostream& out) const {
   switch (tag()) {
     case MemberKind::FieldMember:
       const auto& field = GetFieldMember();
-      out << "var " << field.name << " : " << *field.type << ";\n";
+      out << "var " << field.binding << ";\n";
       break;
   }
 }