RFR: 8372258: Improve TypeVariable support

[Per Minborg]

On Thu, 20 Nov 2025 13:06:12 GMT, Per Minborg <pminborg at openjdk.org> wrote:

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

This PR passes tier1-tier3 tests on multiple platforms.

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PR Comment: https://git.openjdk.org/jdk/pull/28429#issuecomment-3557975090