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
This is the block argument equivalent of the existing `getAsmResultNames` hook.
Closestensorflow/mlir#329
COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/329 from plaidml:flaub-region-arg-names fc7876f2d1335024e441083cd25263fd6247eb7d
PiperOrigin-RevId: 286523299
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