[code-reflection] RFR: Immutble ExternalizedOp.attributes

[Paul Sandoz]

The attributes map of `ExternalizedOp` no longer needs to be immutable. `OpBuilder` is modified accordingly to use `Map.of`.

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Commit messages:
 - Merge branch 'code-reflection' into extern-op-attr-map
 - Mutability not requred.

Changes: https://git.openjdk.org/babylon/pull/407/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=407&range=00
  Stats: 30 lines in 2 files changed: 8 ins; 8 del; 14 mod
  Patch: https://git.openjdk.org/babylon/pull/407.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/407/head:pull/407

PR: https://git.openjdk.org/babylon/pull/407