rust: fix ICE during name resolution for impls on unit-types

rust: fix ICE during name resolution for impls on unit-types

The canonical paths need to support unit-types which are technically a
TupleType with no fields. This handles this case and adds an unreachable.

Fixes #3036

gcc/rust/ChangeLog:

	* resolve/rust-ast-resolve-type.cc (ResolveTypeToCanonicalPath::visit): add unit-type catch
	* resolve/rust-ast-resolve-type.h: likewise

gcc/testsuite/ChangeLog:

	* rust/compile/nr2/exclude: nr2 cant handle this
	* rust/compile/issue-3036.rs: New test.

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