gccrs: Remove unneeded clones untill we have an arena allocator for these tmps

gccrs: Remove unneeded clones untill we have an arena allocator for these tmps

Cloning inference variables is very expensive because it means we are indirectly
creating an implicit new inference variable added to the reference chain.

gcc/rust/ChangeLog:

	* checks/errors/privacy/rust-privacy-reporter.cc (PrivacyReporter::check_base_type_privacy):
	no need for unreachable here
	* typecheck/rust-unify.cc (UnifyRules::commit): dont clone infer vars
	(UnifyRules::expect_inference_variable): likewise

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