rustc: Switch struct fields to private by default

This commit switches privacy's checking of fields to have *all* fields be
private by default. This does not yet change tuple structs, this only affects
structs with named fields. The fallout of this change will follow shortly.

RFC: 0004-private-fields

cc #8122
Closes #11809
std: Switch field privacy as necessary
collections: Switch field privacy as necessary
getopts: Switch field privacy as necessary
native: Switch field privacy as necessary
rand: Switch field privacy as necessary
sync: Switch field privacy as necessary
arena: Switch field privacy as necessary
log: Swith field privacy as necessary
serialize: Switch field privacy as necessary
term: Switch field privacy as necessary
test: Switch field privacy as necessary
syntax: Switch field privacy as necessary
rustc: Switch field privacy as necessary
glob: Switch field privacy as necessary
workcache: Switch field privacy as necessary
rustuv: Switch field privacy as necessary
rustdoc: Switch field privacy as necessary
green: Switch field privacy as necessary
num: Switch field privacy as necessary