gccrs: Use tl::expected in the parser to avoid error state

gccrs: Use tl::expected in the parser to avoid error state

We made heavy use of error state within some AST node and it was the
source of multiple errors, and caused confusion with (null) pointers.
This commit removes some error state use within our parser in an attempt
to remove those error states later.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_inner_attributes): Change
	return type to avoid empty/error values that may break invariants in
	the AST.
	(Parser::parse_inner_attribute): Likewise.
	(Parser::parse_outer_attribute): Likewise.
	(Parser::parse_outer_attributes): Likewise.
	(Parser::parse_attribute_body): Likewise.
	(Parser::parse_simple_path): Likewise.
	(Parser::parse_macro_invocation): Likewise.
	(Parser::parse_visibility): Likewise.
	(Parser::parse_use_tree): Likewise.
	(Parser::parse_delim_token_tree): Likewise.
	(Parser::parse_identifier_or_keyword_token): Likewise.
	(Parser::parse_token_tree): Likewise.
	(Parser::parse_macro_rules_def): Likewise.
	(Parser::parse_decl_macro_def): Likewise.
	(Parser::parse_macro_invocation): Likewise.
	(Parser::parse_macro_rule): Likewise.
	(Parser::parse_macro_matcher): Likewise.
	(Parser::parse_type_path_segment): Likewise.
	(Parser::parse_path_expr_segment): Likewise.
	(Parser::parse_type): Likewise.
	(Parser::parse_type_no_bounds): Likewise.
	(Parser::parse_items): Circumvent GCC 5 to 7 bug.
	(is_simple_path_segment): Move to utility file.
	(token_id_matches_delims): Likewise.
	(is_likely_path_next): Remove unused function.
	(Parser::parse_attr_input): Return a structure instead of a tuple.
	* expand/rust-macro-builtins-offset-of.cc: Adapt call to expected.
	* ast/rust-ast.cc (AttributeParser::parse_path_meta_item): Use empty
	vector when an error is encountered.
	* expand/rust-macro-builtins-include.cc: Likewise.
	* parse/rust-parse.h: Update prototypes.
	* parse/rust-parse-impl-proc-macro.cc: Likewise.
	* ast/rust-ast.h: Remove error state from Visibility.
	* ast/rust-item.h: Use private visibility instead of error.
	* ast/rust-macro.h: Likewise.
	* expand/rust-macro-expand.cc: Likewise.
	* hir/rust-ast-lower.cc: Remove error case.
	* rust-session-manager.cc: Use private visibility
	* parse/rust-parse-utils.h: New file.
	* parse/rust-parse-error.h: New file.

gcc/testsuite/ChangeLog:

	* rust/compile/macros/mbe/macro-issue3608.rs: Update error messages.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>