gccrs: fix parser error on parenthesised types

Do not cast parenthesised types to TraitBound types

Fixes #4148

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h
	(Parser::parse_paren_prefixed_type): remove cast to TraitBound.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4148.rs: test should not produce errors.

Signed-off-by: lenny.chiadmi-delage <lenny.chiadmi-delage@epita.fr>

Author: Villosse