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.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 {
@@ -45,7 +46,7 @@ struct Assign {
 
 struct VariableDefinition {
   static constexpr StatementKind Kind = StatementKind::VariableDefinition;
-  const Expression* pat;
+  const Pattern* pat;
   const Expression* init;
 };
 
@@ -89,7 +90,7 @@ struct Continue {
 struct Match {
   static constexpr StatementKind Kind = StatementKind::Match;
   const Expression* exp;
-  std::list<std::pair<const Expression*, const Statement*>>* clauses;
+  std::list<std::pair<const Pattern*, const Statement*>>* clauses;
 };
 
 struct Continuation {
@@ -113,7 +114,7 @@ struct Statement {
       -> const Statement*;
   static auto MakeAssign(int line_num, const Expression* lhs,
                          const Expression* rhs) -> const Statement*;
-  static auto MakeVariableDefinition(int line_num, const Expression* pat,
+  static auto MakeVariableDefinition(int line_num, const Pattern* pat,
                                      const Expression* init)
       -> const Statement*;
   static auto MakeIf(int line_num, const Expression* cond,
@@ -129,7 +130,7 @@ struct Statement {
   static auto MakeContinue(int line_num) -> const Statement*;
   static auto 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*;
   // Returns an AST node for a continuation statement give its line number and
   // contituent parts.