Stabilize the map/value methods on ControlFlow

[scottmcm]

And fix the stability attribute on the pub use in core::ops.

libs-api in #75744 (comment) seemed reasonably happy with naming for these, so let's try for an FCP.

Summary:

impl<B, C> ControlFlow<B, C> {
    pub fn break_value(self) -> Option<B>;
    pub fn map_break<T>(self, f: impl FnOnce(B) -> T) -> ControlFlow<T, C>;
    pub fn continue_value(self) -> Option<C>;
    pub fn map_continue<T>(self, f: impl FnOnce(C) -> T) -> ControlFlow<B, T>;
}

Resolves #75744

@rustbot label +needs-fcp +t-libs-api -t-libs

---

Aside, in case it keeps someone else from going down the same dead end: I looked at the {break,continue}_value methods and tried to make them const as part of this, but that's disallowed because of not having const Drop, so put it back to not even unstably-const.

[rustbot]

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[jieyouxu]

r? libs-api

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[scottmcm]

Wanted to draw attention to this update specifically. The type has been stable for a while https://blog.rust-lang.org/2021/09/09/Rust-1.55.0.html#stabilized-apis, so I guess this attribute just wasn't doing anything. I set the since to the same old release as the type.