Track a resolved Member on each member access expression. (#1333)

Type checking now stores information on a member access identifying the member that was accessed, not only its name. This parallels what we do for other similar constructs whose meaning is resolved by name lookup or type-checking, and will allow us to avoid redoing lookups in some cases during interpretation.

explorer/ast/declaration.cpp

@@ -143,7 +143,8 @@ void Declaration::PrintID(llvm::raw_ostream& out) const {
   }
 }
 
-auto GetName(const Declaration& declaration) -> std::optional<std::string> {
+auto GetName(const Declaration& declaration)
+    -> std::optional<std::string_view> {
   switch (declaration.kind()) {
     case DeclarationKind::FunctionDeclaration:
       return cast<FunctionDeclaration>(declaration).name();