gccrs: handle bare function types with no specified return type

gccrs: handle bare function types with no specified return type

When we have a function with no return type this defaults to (), but we
need to be sure we generate a new implicit HirId for it otherwise it will
end up in a recursive reference chain.

Fixes #2042

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::visit):
	Add implicit unit type as the return type when not specified

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2042.rs: New test.

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