gccrs: empty match expressions should resolve to !

gccrs: empty match expressions should resolve to !

This is a special case in Rust and the ! type can unify with pretty much
anything its almost a inference variable and a unit-type for special cases.

Fixes Rust-GCC/gccrs#3231
Fixes Rust-GCC/gccrs#2567

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (check_match_scrutinee): check for empty match
	(CompileExpr::visit): fix assertion
	* checks/errors/rust-hir-pattern-analysis.cc (check_match_usefulness): check for empty
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): resolve to !

gcc/testsuite/ChangeLog:

	* rust/compile/exhaustiveness1.rs: remove bad check
	* rust/compile/issue-2567-1.rs: New test.
	* rust/compile/issue-2567-2.rs: New test.
	* rust/compile/issue-2567-3.rs: New test.
	* rust/compile/issue-3231.rs: New test.

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