Make the names of declarations unusable before the point where their type is known. (#1352)

Following #875, diagnose any use of a name prior to the point where it is introduced and its type is known.

This works by performing name resolution on a top-level class, interface, or impl twice: the first pass performs name-resolution for everything other than nested function bodies, and the second pass performs name resolution on function bodies. At the moment, the second pass does a superset of the work done by the first pass, and as a consequence, some identifier expressions now have their target set twice to the same thing.

explorer/ast/static_scope.cpp

@@ -21,6 +21,7 @@ auto StaticScope::Add(const std::string& name, ValueNodeView entity,
     CARBON_CHECK(usable || !it->second.usable)
         << entity.base().source_loc() << " attempting to mark a usable name `"
         << name << "` as unusable";
+    it->second.usable |= usable;
   }
   return Success();
 }