move async unsafe fn to distinct feature gate

[nikomatsakis]

Per @Centril's comment, we want to move async unsafe fn to its own feature gate:

I would also like to move async unsafe fn out of the MVP into its own feature gate because I think a) it has seen little use, b) it is not particularly well tested, c) it ostensibly behaves weirdly because the .await point is not where you write unsafe { ... } and this is understandable from "leaky implementation POV" but not so much from an effects POV, d) it has seen little discussion and was not included in the RFC nor this report, and e) we did this with const fn and it worked fine. (I can write up the feature gating PR)

This issue has been assigned to @delan via this comment.

[Centril]

Mentoring instructions:

1. In libsyntax/parse/mod.rs there's a struct ParseSess. To it, you want to add: pub async_unsafe_spans: Lock<Vec<Span>>, and then modify fn with_span_handler as well in the same file.

2. In libsyntax/feature_gate.rs you want to add a new active feature gate to the bottom (see the async_closure example) and then modify check_crate (also see async_closure_spans example).

3. In parse_fn_front_matter inside libsyntax/parse/parser.rs you want to modify fn parse_fn_front_matter such that when asyncness.is_async() and unsafety == Unsafety::Unsafe coincide, you push the Span of the unsafe token to async_unsafe_spans.

4. In parse_item_implementation inside libsyntax/parse/parser.rs you want to modify the if self.check_keyword(kw::Async) { case analogously.

5. You then want to add a feature gate test to src/test/ui/async-await/feature-async-unsafe.rs which also checks that the feature gate is required even under #[cfg(FALSE)] on a function. Make sure you test both free functions and functions in implementations.

6. You will also need to edit various tests which will now fail due to feature gating. Make sure to also move out async unsafe fn tests into standalone test files.

For an example of splitting a feature gate see #62292.

[Centril]

The tracking issue to use is #62501.

[delan]

@rustbot claim

[Centril]

I was convinced not to feature gate this after all (see #62149 (comment)). Sorry about the noise!