Rename the file permission statics in std::io to be uppercase

For example, this renames `GroupRWX` to `GROUP_RWX`, and deprecates the old
name. Code using these statics should be updated accordingly.
Remove the #[allow(non_uppercase_statics)] attr from bitflags!
auto merge of #17781 : P1start/rust/bitflags-lints, r=alexcrichton

Closes #17773.