gccrs: Fix ICE when typechecking non-trait item when we expect one

gccrs: Fix ICE when typechecking non-trait item when we expect one

We just had an assertion here for this case where we expect a trait.
This changes the assertion into error handling producing the correct
error code with fixit suggestion like rustc.

Fixes #2499

gcc/rust/ChangeLog:

	* typecheck/rust-hir-trait-resolve.cc (TraitResolver::resolve_path_to_trait):
	use error handling instead of assertion
	* typecheck/rust-hir-type-check-item.cc (TypeCheckItem::visit): reuse trait reference
	* typecheck/rust-hir-type-check-item.h: update prototype

gcc/testsuite/ChangeLog:

	* rust/compile/nr2/exclude: nr2 cant handle this
	* rust/compile/issue-2499.rs: New test.

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