gccrs: diagnostic: Refactor Error class

The class now allows for more variants including a `Hint` one which
then gets emitted by calling `rust_inform`. This allows us to display
hints/tips/notes in backtracking contexts such as the parser.

gcc/rust/ChangeLog:

	* rust-diagnostics.h (struct Error): Add new Kind enum and various new
	static constructors to allow for hints as well.
	* rust-diagnostics.cc (Error::Error): Use new `kind` field properly.
	* checks/errors/privacy/rust-visibility-resolver.cc
	(VisibilityResolver::resolve_module_path): Use new Error API.
	* expand/rust-macro-builtins.cc (MacroBuiltin::include_handler): Likewise.
	* expand/rust-macro-expand.cc (parse_many): Likewise.
	(transcribe_type): Likewise.
	* parse/rust-parse-impl.h (Parser::parse_crate): Likewise.
	* rust-session-manager.cc (Session::handle_crate_name): Likewise.
	* ast/rust-ast.cc (Module::load_items): Likewise.