[AutoDiff] Enable cross-file derivative registration by default.

[dan-zheng]

Enable cross-file derivative registration by default on tensorflow branch.
Cross-file derivative registration is already enabled on master branch.

Remove -enable-experimental-cross-file-derivative-registration flag, which was
never upstreamed to master.

Resolves TF-1097.

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This brings tensorflow in sync with master.

Todos:

• Upstream cross-file derivative registration tests: from test/AutoDiff/downstream and abandoned PR [AutoDiff] lift samefile derivative constriant #28790.
• Require same access level for original/derivative functions: [AutoDiff] Require same access level for original/derivative functions. #29987.
  • This tightens derivative registration typing rules, since we decided not to pursue "private derivatives" for now (given that there are no use cases, and that private conformances are not even implemented).

[dan-zheng]

@swift-ci Please test tensorflow

[rxwei]

Did we land #29987 on master? I think there's quite a few stability issues with cross-file derivative registration with mismatching access levels. It would be better if #29987 lands first on master. But since this PR is about the tensorflow branch, I'll let you guys decide.

[dan-zheng]

Did we land #29987 on master? I think there's quite a few stability issues with cross-file derivative registration with mismatching access levels. It would be better if #29987 lands first on master. But since this PR is about the tensorflow branch, I'll let you guys decide.

#29987 hasn't landed on master yet, I'll look into it soon to address potential issues from mismatching access levels. This PR just makes tensorflow branch consistent with master.