Don't invoke dataConSrcToImplBang on newtypes
Trac #16141 was caused by StrictData sneaking in some unwanted unpacking on the field of a newtype due to the dataConSrcToImplBang function (which really only makes sense for data types) always being invoked from mkDataConRep. Solution: don't invoke dataConSrcToImplBang when dealing with newtypes.