RFR: JDK-8315137: Add explicit override RecordComponentElement.asType()

[Joe Darcy]

No change in semantics, just adding more explicit information.

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Commit messages:
 - JDK-8315137: Add explicit override RecordComponentElement.asType()

Changes: https://git.openjdk.org/jdk/pull/15456/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315137
  Stats: 17 lines in 2 files changed: 16 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15456/head:pull/15456

PR: https://git.openjdk.org/jdk/pull/15456