gccrs: Extract helper lookup_associated_impl_block

gccrs: Extract helper lookup_associated_impl_block

This will look for a specified predicate that is associated with it so we
might have the predicate Foo<u16, i32> which would refer to:

  impl Foo<i32> for u16 {}

This is a general helper which can be used in several places.

gcc/rust/ChangeLog:

	* typecheck/rust-hir-trait-reference.cc (AssociatedImplTrait::AssociatedImplTrait):
	bind the predicate
	(AssociatedImplTrait::get_trait): remove
	(AssociatedImplTrait::get_predicate): new getter
	* typecheck/rust-hir-trait-reference.h: bind predicate
	* typecheck/rust-hir-type-check-item.cc: update ctor
	* typecheck/rust-type-util.cc (lookup_associated_impl_block): new function
	* typecheck/rust-type-util.h (class BaseType): remove unused forward decl
	(lookup_associated_impl_block): new helper
	* typecheck/rust-tyty-bounds.cc (TypeBoundPredicate::is_equal): new
	* typecheck/rust-tyty-subst.cc (SubstitutionRef::lookup_associated_impl): use new helper
	(SubstitutionRef::monomorphize): update usage/error handling
	* typecheck/rust-tyty-subst.h: remove old prototype
	* typecheck/rust-tyty.h: add is_equal decl

Signed-off-by: Philip Herron <herron.philip@googlemail.com>