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/interpreter/interpreter.h

@@ -11,6 +11,8 @@
 
 #include "common/ostream.h"
 #include "executable_semantics/ast/declaration.h"
+#include "executable_semantics/ast/expression.h"
+#include "executable_semantics/ast/pattern.h"
 #include "executable_semantics/interpreter/frame.h"
 #include "executable_semantics/interpreter/heap.h"
 #include "executable_semantics/interpreter/stack.h"
@@ -35,6 +37,7 @@ void PrintEnv(Env values);
 
 auto InterpProgram(std::list<Declaration>* fs) -> int;
 auto InterpExp(Env values, const Expression* e) -> const Value*;
+auto InterpPattern(Env values, const Pattern* p) -> const Value*;
 
 }  // namespace Carbon