[code-reflection] RFR: Immutble ExternalizedOp.attributes [v3]

[Paul Sandoz]

> The attributes map of `ExternalizedOp` no longer needs to be immutable. `OpBuilder` is modified accordingly to use `Map.of`.
> 
> In addition `OpBuilder` is also modified to reuse previously constructed type elements.

Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:

  Remove special constructor (revisit later).

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Changes:
  - all: https://git.openjdk.org/babylon/pull/407/files
  - new: https://git.openjdk.org/babylon/pull/407/files/40650ec0..de14df0e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=407&range=02
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=407&range=01-02

  Stats: 31 lines in 2 files changed: 0 ins; 23 del; 8 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