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/statement.cpp

@@ -76,7 +76,7 @@ auto Statement::MakeAssign(int line_num, const Expression* lhs,
   return s;
 }
 
-auto Statement::MakeVariableDefinition(int line_num, const Expression* pat,
+auto Statement::MakeVariableDefinition(int line_num, const Pattern* pat,
                                        const Expression* init)
     -> const Statement* {
   auto* s = new Statement();
@@ -142,7 +142,7 @@ auto Statement::MakeBlock(int line_num, const Statement* stmt)
 
 auto Statement::MakeMatch(
     int line_num, const Expression* exp,
-    std::list<std::pair<const Expression*, const Statement*>>* clauses)
+    std::list<std::pair<const Pattern*, const Statement*>>* clauses)
     -> const Statement* {
   auto* s = new Statement();
   s->line_num = line_num;