rustc: Relax restriction on privacy for fields

This is a necessary change in preparation for switching the defaults as part
of #8122.

RFC: 0004-private-fields
syntax: Permit visibility on tuple fields

This change is in preparation for #8122. Nothing is currently done with these
visibility qualifiers, they are just parsed and accepted by the compiler.

RFC: 0004-private-fields
auto merge of #13145 : alexcrichton/rust/flip-some-defaults, r=brson

This change prepares `rustc` to accept private fields by default. These changes will have to go through a snapshot before the rest of the changes can happen.