Generated vars shouldn't shadow existing vars in scope

[zachallaun]

ref = make_ref()

auto_assert make_ref()

# currently generates
auto_assert ref when is_reference(ref) <- make_ref()

# should generate
auto_assert ref1 when is_reference(ref1) <- make_ref()

Should be an easy fix. We have all the info we need in context when building patterns, just need to write a make_unique_var/2 instead of make_var/2 that use that when generating guard patterns.