[mlir] Drop the leading space when printing regions

The leading space that is always printed at the beginning of regions is not consistent with other parts of the printing API. Moreover, this leading space can lead to undesirable assembly formats:

```
attr-dict-with-keyword $region
```

Prints as:

```
// Two spaces between `}` and `{`
attributes {foo}  { ... }
```

Moreover, the leading space results in the odd generic op format:

```
"test.op"() ( {...}) : () -> ()
```

Reviewed By: rriddle, mehdi_amini

Differential Revision: https://reviews.llvm.org/D117411