gccrs: filter out candidates by checking if self is compatable

We have PartialOrd for &i8 and a reference &&A so we need to filter out the
double reference one early on.

Fixes Rust-GCC/gccrs#4866

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-expr.cc: check candidate self

gcc/testsuite/ChangeLog:

	* rust/execute/torture/issue-4866.rs: New test.

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