typecheck: Properly select methods when dealing with specialization

typecheck: Properly select methods when dealing with specialization

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-expr.cc (is_default_fn): New.
	(emit_ambiguous_resolution_error): New.
	(handle_multiple_candidates): Properly handle multiple candidates in
	the case of specialization.
	(TypeCheckExpr::visit): Call `handle_multiple_candidates`.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/min_specialization2.rs: New test.
	* rust/execute/torture/min_specialization3.rs: New test.