Rollup merge of #69201 - Aaron1011:feature/permit-if-attr, r=Centril

Permit attributes on 'if' expressions

Previously, attributes on 'if' expressions (e.g. `#[attr] if true {}`)
were disallowed during parsing. This made it impossible for macros to
perform any custom handling of such attributes (e.g. stripping them
away), since a compilation error would be emitted before they ever had a
chance to run.

This PR permits attributes on 'if' expressions ('if-attrs' from here on).
Both built-in attributes (e.g. `#[allow]`, `#[cfg]`) and proc-macro attributes are supported.

We still do *not* accept attributes on 'other parts' of an if-else
chain. That is, the following code snippet still fails to parse:

```rust
if true {} #[attr] else if false {} else #[attr] if false {} #[attr]
else {}
```

Closes https://github.com/rust-lang/rust/issues/68618

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