Auto merge of #139087 - beetrees:impl-from-f16-for-f32, r=jackh726

Fallback `{float}` to `f32` when `f32: From<{float}>` and add `impl From<f16> for f32`



Currently, the following code compiles:
```rust
fn foo<T: Into<f32>>(_: T) {}

fn main() {
    foo(1.0);
}
```

This is because the only `From<{float}>` impl for `f32` is currently `From<f32>`. However, once `impl From<f16> for f32` is added this is no longer the case. This would cause the float literal to fallback to `f64`, subsequently causing a type error as `f32` does not implement `From<f64>`. While this kind of change to type inference isn't technically a breaking change according to Rust's breaking change policy, the previous attempt to add `impl From<f16> for f32` was removed rust-lang/rust#123830 due to the large number of crates affected (by my count, there were root regressions in 42 crates and 52 GitHub repos, not including duplicates). This PR solves this problem by using `f32` as the fallback type for `{float}` when there is a trait predicate of `f32: From<{float}>`. This allows adding `impl From<f16> for f32` without affecting the code that currently compiles (such as the example above; this PR shouldn't affect what is possible on stable).

This PR also allows adding a future-incompatibility warning for the fallback to `f32` (currently implemented in the third commit) if the lang team wants one (allowing the `f32` fallback to be removed in the future); alternatively this could be expanded in the future into something more general like @tgross35 suggested in https://github.com/rust-lang/rust/issues/123831#issuecomment-2064728053. I think it would be also possible to disallow the `f32` fallback in a future edition.

As expected, a crater check showed [no non-spurious regressions](https://github.com/rust-lang/rust/pull/139087#issuecomment-2764732580).

For reference, I've based the implementation loosely on the existing `calculate_diverging_fallback`. This first commit adds the `f32` fallback, the second adds `impl From<f16> for f32`, and the third adds a FCW lint for the `f32` fallback. I think this falls under the types team, so
r? types

Fixes: rust-lang/rust#123831
Tracking issue: rust-lang/rust#116909

@rustbot label +T-lang +T-types +T-libs-api +F-f16_and_f128

To decide on whether a future-incompatibility warning is desired or otherwise (see above):
@rustbot label +I-lang-nominated

cc https://github.com/rust-lang/rust/issues/154024 https://github.com/rust-lang/rust/issues/154005