d: Cache generated import declarations in a hash_map
Originally, these were cached in the front-end AST node field `isym'.
However, this field is due to be removed in the future.
gcc/d/ChangeLog:
* imports.cc (imported_decls): Define.
(class ImportVisitor): Add result_ field.
(ImportVisitor::result): New method.
(ImportVisitor::visit (Module *)): Store decl to result_.
(ImportVisitor::visit (Import *)): Likewise.
(ImportVisitor::visit (AliasDeclaration *)): Don't cache decl in
front-end AST node.
(ImportVisitor::visit (OverDeclaration *)): Likewise.
(ImportVisitor::visit (FuncDeclaration *)): Likewise.
(ImportVisitor::visit (Declaration *)): Likewise.
(build_import_decl): Use imported_decls to cache and lookup built
declarations.