[clang] Instantiate concepts with sugared template arguments

Since we don't unique specializations for concepts, we can just instantiate
them with the sugared template arguments, at negligible cost.

If we don't track their specializations, we can't resugar them later
anyway, and that would be more expensive than just instantiating them
sugared in the first place since it would require an additional pass.

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

Differential Revision: https://reviews.llvm.org/D136566