gccrs: Fix invalid call to vector::front in TypeBoundPredicate constructors

gccrs: Fix invalid call to vector::front in TypeBoundPredicate constructors

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-expr.cc
	(TypeCheckExpr::resolve_fn_trait_call): Add TODO comment.
	* typecheck/rust-tyty-bounds.cc
	(TypeBoundPredicate::TypeBoundPredicate):
	Add assertions, new error constructor.
	(TypeBoundPredicate::error): Use new error constructor.
	* typecheck/rust-tyty.h
	(struct TypeBoundPredicate::mark_is_error): New.
	(TypeBoundPredicate::TypeBoundPredicate):
	Add new error constructor.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>