Pending further discussion, re-enable warnings for Objective C

covariant/contravariant overrides and implementations, but do so under
control of a new flag (-Wno-objc-covariant-overrides, which yes does cover
contravariance too).

*At least* the covariance cases will probably be enabled by default shortly,
but that's not totally uncontroversial.

llvm-svn: 117346