[code-reflection] RFR: OnnxProtoBuilder fixes

[Adam Sotona]

- fixes ONNX graph output name of returned tuple element
- fixes String AttributeProto value

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Commit messages:
 - OnnxProtoBuilder fixes

Changes: https://git.openjdk.org/babylon/pull/614/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=614&range=00
  Stats: 7 lines in 1 file changed: 6 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/614.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/614/head:pull/614

PR: https://git.openjdk.org/babylon/pull/614