[clang] CTAD: fix the aggregate deduction guide for alias templates. (#90894)

For alias templates, our current way of constructing their aggregate
deduction guides deviates from the standard approach. We should align it
with how we handle implicit deduction guides.

This patch has a refactoring change which pulls the construction logic
out from `DeclareImplicitDeductionGuidesForTypeAlia` and reusing it for
building aggregate deduction guides.