gccrs: Fix ICE in get_function_expr when cfg'd return type inside macro

the problem is cfg-strip emits an error for unstrippable expressions but
doesn't mark the parent for strip, leaving a broken subtree for later
passes to ICE on.

gcc/rust/ChangeLog:

	* expand/rust-cfg-strip.cc (CfgStrip::visit): mark CallExpr for
	strip when function expression fails stripping.
	(CfgStrip::visit): mark ArrayIndexExpr for strip when array or
	index expression fails stripping.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4167.rs: New test.

Signed-off-by: Harishankar <harishankarpp7@gmail.com>