[code-reflection] RFR: Immediate dominator map should be unmodifiable

[Paul Sandoz]

The cached Immediate dominator map returned by `Body::immediateDominators` was modifiable. Changed to make it unmodifiable.

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Commit messages:
 - Immediate dominator map should be unmodifiable

Changes: https://git.openjdk.org/babylon/pull/187/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=187&range=00
  Stats: 31 lines in 2 files changed: 26 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/babylon/pull/187.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/187/head:pull/187

PR: https://git.openjdk.org/babylon/pull/187