gccrs: Fix compilation of types which hold onto dangling infer vars

gccrs: Fix compilation of types which hold onto dangling infer vars

There is a case where some generic types are holding onto inference
variable pointers directly. So this gives the backend a chance to do one
final lookup to resolve the type.

This now allows us to compile a full test case for iterators but there is
still one miscompilation in here which results in a segv on O2 and bad
result on -O0.

Addresses #1895

gcc/rust/ChangeLog:

	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): do a final lookup

gcc/testsuite/ChangeLog:

	* rust/compile/iterators1.rs: New test.

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