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

[philberty]

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.

test.rs:7:6: error: Expected a trait found ‘Foo’ [E0404]
    7 | impl Foo for Bar {}
      |      ^~~
      |      not a trait
test.rs:10:11: error: Expected a trait found ‘Foo’ [E0404]
   10 | fn baz<T: Foo>(t: T) {}
      |           ^~~
      |           not a trait

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.