Replace some keyword raw values

Replace some keyword raw values

Raw values cannot be understood easily by most tools. This commit replace
some raw values with their variable counterpart.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Replace raw value
	with keyword call.
	* ast/rust-ast.h: Likewise.
	* parse/rust-parse-impl.h (Parser::parse_path_ident_segment): Likewise.
	(Parser::parse_macro_match_fragment): Likewise.
	(Parser::parse_extern_crate): Likewise.
	(Parser::parse_use_tree): Likewise.
	(Parser::parse_const_item): Likewise.
	(Parser::parse_literal_expr): Likewise.
	(Parser::parse_maybe_named_param): Likewise.
	(Parser::parse_pattern_no_alt): Likewise.
	(Parser::left_denotation): Likewise.
	(Parser::parse_path_in_expression_pratt): Likewise.

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