Rollup merge of #154057 - aytey:fix-block-index-paren, r=fmease

Parenthesize block-like expressions in index base of pretty printer

The AST pretty printer produces invalid Rust when a block expression is the base of an index operation inside a macro expansion. This is a gap in the parenthesization fix from rust-lang/rust#119105 — the `FixupContext` approach handles statement position but not the case where a block-index is nested inside another expression.

The following is a correct program:

```rust
macro_rules! block_arr {
    () => {{ [0u8; 4] }};
}

macro_rules! as_slice {
    () => {{ &block_arr!()[..] }};
}

fn main() { let _: &[u8] = as_slice!(); }
```

But `rustc -Zunpretty=expanded` produces output that is not valid Rust, because the closing brace of `{ [0u8; 4] }` creates a statement boundary, causing the parser to treat `[..]` as a separate expression:

```rust
fn main() { let _: &[u8] = { &{ [0u8; 4] }[..] }; }
```

```
error: expected one of `.`, `;`, `?`, `}`, or an operator, found `[`
```

Fixed output after this change:

```rust
fn main() { let _: &[u8] = { &({ [0u8; 4] })[..] }; }
```

Since `{ ... }[...]` never parses as indexing a block regardless of context, the fix unconditionally parenthesizes "complete" expressions (block, match, if, loop, etc.) when they appear as the base of an index operation.