cfg attribute should remove the entire item from type checking, including its annotations

[ironcev]

By definition, a cfg attribute that cfg-evals to false effectively removes its annotated item from the AST and further type checking. E.g., this compiles when new_encoding is active, because the whole function gets removed from the type checking:

#[cfg(experimental_new_encoding = false)]
fn not_type_checked() -> bool {
    non_existing + non_existing
}

Since the item is removed, its annotations must be removed as well. E.g., we shouldn't get warnings or errors for invalid annotations. This is currently not the case. E.g., this code:

#[allow(something)]        // <<<<---- Emits warning here, although `not_type_checked` does not exist.
#[cfg(experimental_new_encoding = false)]
fn not_type_checked() -> bool {
    non_existing + non_existing
}

will emit a warning, although the not_type_checked does not exist in the further compilation.