Move pattern interpretation to compile time (#904)

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/ast/pattern.h

@@ -60,6 +60,18 @@ class Pattern {
   // and after typechecking it's guaranteed to be true.
   auto has_static_type() const -> bool { return static_type_.has_value(); }
 
+  // The value of this pattern. Cannot be called before typechecking.
+  auto value() const -> const Value& { return **value_; }
+
+  // Sets the value of this pattern. Can only be called once, during
+  // typechecking.
+  void set_value(Nonnull<const Value*> value) { value_ = value; }
+
+  // Returns whether the value has been set. Should only be called
+  // during typechecking: before typechecking it's guaranteed to be false,
+  // and after typechecking it's guaranteed to be true.
+  auto has_value() const -> bool { return value_.has_value(); }
+
  protected:
   // Constructs a Pattern representing syntax at the given line number.
   // `kind` must be the enumerator corresponding to the most-derived type being
@@ -72,6 +84,7 @@ class Pattern {
   SourceLocation source_loc_;
 
   std::optional<Nonnull<const Value*>> static_type_;
+  std::optional<Nonnull<const Value*>> value_;
 };
 
 // A pattern consisting of the `auto` keyword.