Expect parentheses after an alternative only if they were present in its declaration (#2605)

The design of choice types expects the declaration of an alternative to match the usage: if an alternative is declared as `None`, then it should be used as `None` not `None()`, and if it is declared as `None()` then it should be used as `None()` not `None`. Update explorer to match.

Also clean up the handling of choice types and alternative values a little in general, by moving away from identifying choice types and alternatives as strings and towards identifying them symbolically.

Closes #2422

explorer/ast/declaration.cpp

@@ -429,11 +429,24 @@ auto ImplDeclaration::Create(Nonnull<Arena*> arena, SourceLocation source_loc,
 }
 
 void AlternativeSignature::Print(llvm::raw_ostream& out) const {
-  out << "alt " << name() << " " << signature();
+  out << "alt " << name();
+  if (auto sig = signature()) {
+    out << **signature();
+  }
 }
 
 void AlternativeSignature::PrintID(llvm::raw_ostream& out) const {
   out << name();
 }
 
+auto ChoiceDeclaration::FindAlternative(std::string_view name) const
+    -> std::optional<const AlternativeSignature*> {
+  for (auto* alt : alternatives()) {
+    if (alt->name() == name) {
+      return alt;
+    }
+  }
+  return std::nullopt;
+}
+
 }  // namespace Carbon