gccrs: parser: Improve parsing of complex generic arguments

gccrs: parser: Improve parsing of complex generic arguments

The parser was missing code for handling complex type arguments such
as type paths or nested generics.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_generic_arg): Handle type
	paths and nested generics properly.

gcc/testsuite/ChangeLog:

	* rust/compile/parse_complex_generic_application.rs: New test.
	* rust/compile/parse_complex_generic_application2.rs: New test.