Handle use-before-declare in static name lookup (#967)

Also remove inheritance from NamedEntity for some classes that don't need it.

executable_semantics/ast/ast.h

@@ -9,7 +9,6 @@
 
 #include "executable_semantics/ast/declaration.h"
 #include "executable_semantics/ast/library_name.h"
-#include "executable_semantics/ast/static_scope.h"
 #include "executable_semantics/common/nonnull.h"
 
 namespace Carbon {
@@ -24,8 +23,6 @@ struct AST {
   std::vector<LibraryName> imports;
   // The file's ordered declarations.
   std::vector<Nonnull<Declaration*>> declarations;
-  // Names declared at the top level of the file.
-  StaticScope static_scope;
   // Synthesized call to `Main`. Injected after parsing.
   std::optional<Nonnull<CallExpression*>> main_call;
 };