ast: Add new GlobContainer class and use it

[CohenArthur]

Allow crates to be used as targets for glob imports, like modules and enums.

gcc/rust/ChangeLog:

* ast/rust-ast-full-decls.h (class GlobContainer): New.
* ast/rust-ast.h (class GlobContainer): New.
(struct Crate): Inherit from GlobContainer.
* ast/rust-item.h (class Module): Likewise.
(class Enum): Likewise.
* resolve/rust-finalize-imports-2.0.cc (GlobbingVisitor::go): Handle crates.
(GlobbingVisitor::visit_crate_container): New.
(GlobbingVisitor::visit): Remove privacy check as it is wrong.
* resolve/rust-finalize-imports-2.0.h: Declare new methods.
* resolve/rust-toplevel-name-resolver-2.0.cc (TopLevel::go): Handle crates.
(TopLevel::visit): Use new insert_glob_container API.
* util/rust-hir-map.cc (Mappings::get_ast_crate_by_node_id_raw): New private method.
(Mappings::get_ast_crate_by_node_id): Use it.
(Mappings::insert_glob_container): New API.
* util/rust-hir-map.h: Likewise.

It's a bit hard to add tests for this, as it requires a bit of a complex file structure that I think we don't handle in the testsuite yet. In any case, it's required for core and it's not destructive changes or anything