[analyzer] Fix inference for variant parameters leaking UnknownTypes.
[analyzer] Fix inference for variant parameters leaking UnknownTypes.

Equivalent CFE fix: https://dart-review.googlesource.com/c/sdk/+/128771
Allows variance_unconstrained_inference_test to pass in analyzer and
fixes issue with the inference leaking UnknownTypes.

Change-Id: Iac00505c06f4a0691cd5a5f5421b9cd102e0636c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/129144
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Kallen Tu <b6f9eba4f92d9a364b1472e1f41eb410080a3b01@google.com>

dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk