gccrs: Remove redundant error types

gccrs: Remove redundant error types

Some error types in the parser had the exact same meaning and could be
grouped under one same type.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (AttributeParser::parse_meta_item_lit): Use
	tl::expected
	* expand/rust-macro-builtins-helpers.cc (parse_single_string_literal):
	Likewise.
	* expand/rust-macro-expand.cc (MacroExpander::match_fragment):
	Likewise.
	* parse/rust-cfg-parser.cc (parse_cfg_option): Likewise.
	* parse/rust-parse-error.h (struct SimplePath): Remove error type.
	(struct DelimTokenTree): Likewise.
	(struct Token): Likewise.
	(struct TokenTree): Likewise.
	(class LifetimeParam): Move from here ...
	(struct LifetimeParam): ... to here. Add ctor.
	(class Lifetime): Add error type.
	(enum class): Remove AnonConst.
	(struct BlockExpr): Change BlockExpr to generic node error.
	* parse/rust-parse-impl-expr.hxx: Use tl::expected for errors.
	* parse/rust-parse-impl-path.hxx: Likewise.
	* parse/rust-parse-impl-ttree.hxx: Likewise.
	* parse/rust-parse-impl.hxx: Likewise.
	* parse/rust-parse.h: Update function return types with tl::expected
	to propagate errors.

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