(PR46111) Properly handle elaborated types in an implicit deduction guide

As reported in PR46111, implicit instantiation of a deduction guide
causes us to have an elaborated type as the parameter, rather than the
dependent type.

After review and feedback from @rsmith, this patch solves this problem
by wrapping the value in an uninstantiated typedef/type-alias that is
instantiated when required later.

Differential Revision: https://reviews.llvm.org/D80743