[clang] Allow pack expansions when partial ordering against template template parameters

When partial ordering alias templates against template template parameters,
allow pack expansions when the alias has a fixed-size parameter list.

These expansions were generally disallowed by proposed resolution for CWG1430.

By previously diagnosing these when checking template template parameters, we
would be too strict in trying to prevent any potential invalid use.

This flows against the more general idea that template template parameters are
weakly typed, that we would rather allow an argument that might be possibly
misused, and only diagnose the actual misuses during instantiation.

Since this interaction between P0522R0 and CWG1430 is also a backwards-compat
breaking change, we implement provisional wording to allow these.

Fixes https://github.com/llvm/llvm-project/issues/62529