Enforce min_const_fn by default in staged_api with opt-out

[Centril]

We want to switch to a scheme where everything inside the standard library requires min_const_fn by default. If an additional #[rustc_not_min_const_fn] is present, then the function will be uncallable from any min_const_fn.

It's also important to note that a function #[rustc_const_unstable(feature "foo")] const fn bar() {} without #[rustc_not_min_const_fn] should a) enforce that that the function is min_const_fn even without the feature gate active, b) not be callable as min_const_fn without the the feature gate active.

Relevant code exists in:

• https://github.com/rust-lang/rust/blob/master/src/librustc/ty/constness.rs
• https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/qualify_min_const_fn.rs
• https://github.com/rust-lang/rust/blob/master/src/librustc_mir/transform/qualify_consts.rs

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Context: Centril@93ad760#r34994115 (also see my branch master...Centril:stabilize-vec-new-const)

Conversation replayed (keep-safe):

@oli-obk:

One thing that could be done is what's essentially the opposite scheme. Everything requires min_const_fn by default, unless it has an additional rustc_not_min_const_fn attribute (which makes the function uncallable from any min_const_fn).

Or rename the attribute to rustc_require_min_const_fn, to make it sound less like an override

@Centril:

One thing that could be done is what's essentially the opposite scheme.

Oh... I like that! -- but what about user-land const fn? Do we only enforce this for staged_api crates?

@oli-obk:

user land const fn just keeps doing what it does now. As long as there's a const fn feature gate active, everything is not min_const_fn. The reason we have a complex scheme in libstd is stability. Users using nightly with feature gates already opt out of stability.

[Centril]

Fixed by @oli-obk in #67136.