[clang] Instantiate alias templates with sugar

[clang] Instantiate alias templates with sugar

This makes use of the changes introduced in D134604, in order to
instantiate alias templates witn a final sugared substitution.

This comes at no additional relevant cost.
Since we don't track / unique them in specializations, we wouldn't be
able to resugar them later anyway.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>
Differential Revision: https://reviews.llvm.org/D136565