Improve hir_pretty for struct expressions.
While working on https://github.com/rust-lang/rust/pull/139131 I noticed the hir pretty printer outputs an empty line between each field, and is also missing a space before the `{` and the `}`:
```rust
let a =
StructWithSomeFields{
field_1: 1,
field_2: 2,
field_3: 3,
field_4: 4,
field_5: 5,
field_6: 6,};
let a = StructWithSomeFields{ field_1: 1, field_2: 2, ..a};
```
This changes it to:
```rust
let a =
StructWithSomeFields {
field_1: 1,
field_2: 2,
field_3: 3,
field_4: 4,
field_5: 5,
field_6: 6 };
let a = StructWithSomeFields { field_1: 1, field_2: 2, ..a };
```
f07a0d117f