Use resolved names in typechecker (#974)

As a byproduct, replace NamedEntity with a type-erasing wrapper NamedEntityView, eliminate virtual inheritance from the AST, and eliminate interfaces from gen_rtti.

executable_semantics/ast/pattern.cpp

@@ -89,7 +89,7 @@ static auto RequireFieldAccess(Nonnull<Expression*> alternative)
 AlternativePattern::AlternativePattern(SourceLocation source_loc,
                                        Nonnull<Expression*> alternative,
                                        Nonnull<TuplePattern*> arguments)
-    : AstNode(AstNodeKind::AlternativePattern, source_loc),
+    : Pattern(AstNodeKind::AlternativePattern, source_loc),
       choice_type_(&RequireFieldAccess(alternative).aggregate()),
       alternative_name_(RequireFieldAccess(alternative).field()),
       arguments_(arguments) {}