gccrs: parser: Allow parsing multiple reference types

gccrs: parser: Allow parsing multiple reference types

The parser now recursively tries to parse a reference type after seeing
a `&` or `&&` token.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_type): Handle double ampersan
	properly
	(Parser::parse_reference_type): Call into `parse_reference_type_inner`
	and wrap double reference types in another `AST::ReferenceType` node
	(Parser::parse_reference_type_inner): Add parsing implementation
	which does not care about the leading token (& or  &&)
	(Parser::parse_type_no_bounds): Handle double ampersand properly
	* parse/rust-parse.h: Declare `parse_reference_type_inner`

gcc/testsuite/ChangeLog:

	* rust/compile/multi_reference_type.rs: New test.