RFR: 8372258: Improve TypeVariable support

[Per Minborg]

The `TypeVariableImpl::getGenericDeclaration` method should return copies of generic declarations where appropriate.

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Commit messages:
 - Creat copies of mutable generic declarations

Changes: https://git.openjdk.org/jdk/pull/28429/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372258
  Stats: 82 lines in 2 files changed: 78 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/28429.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28429/head:pull/28429

PR: https://git.openjdk.org/jdk/pull/28429