rework handling of doc attributes on macro calls

[mejrs]

This PR implements the following

• move feature gating of doc attributes to attribute parsing
• remove OmitDoc (only used for doc expressions on macro calls..?)
• the following lint is now emitted:

// accidentally stabilized in 1.94
#[doc = {
    let a = 1;
    let b = 1;
    let sum = a + b;
    assert_eq!(sum, 2);
}]
foo!();

warning: arbitrary expressions in doc attributes were previously accepted by the compiler
but are being phased out; it will become a hard error in a future release!
   |
LL |       #[doc = {
   |  _____________^
LL | |         let a = 1;
LL | |         let b = 1;
LL | |         let sum = a + b;
LL | |         assert_eq!(sum, 2);
LL | |     }]
   | |_____^

I'd like to try to error on that; I'll do a crater run at some point in the future.

r? @JonathanBrouwer

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.