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.h

@@ -9,6 +9,7 @@
 
 #include "common/ostream.h"
 #include "executable_semantics/ast/expression.h"
+#include "executable_semantics/ast/pattern.h"
 #include "llvm/Support/Compiler.h"
 
 namespace Carbon {
@@ -17,13 +18,15 @@ enum class MemberKind { FieldMember };
 
 struct FieldMember {
   static constexpr MemberKind Kind = MemberKind::FieldMember;
-  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;
 };
 
 struct Member {
-  static auto MakeFieldMember(int line_num, std::string name,
-                              const Expression* type) -> Member*;
+  static auto MakeFieldMember(int line_num, const BindingPattern* binding)
+      -> Member*;
 
   auto GetFieldMember() const -> const FieldMember&;