[clang] Perform sugared substitution of builtin templates

Since these are much like template type aliases, where we don't
track a specialization for them and just substitute them eagerly,
we can't resugar them anyway, and there is no relevant cost in just
performing a finalizing sugared substitution.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D136563