[swift2objc] Support globals with explicit getters/setters

[liamappelbe]

Before this change, variables with an explicit getter but no setter would be wrapped by a getter/setter pair, and the setter was causing compile errors.

[coveralls]

coverage: 90.196% (+0.001%) from 90.195%
when pulling c5184b9 on globalgetter
into 8165cbb on main.