Revert "DR2064: decltype(E) is only a dependent type if E is type-dependent, not

Revert "DR2064: decltype(E) is only a dependent type if E is type-dependent, not
if E is merely instantiation-dependent."

This change leaves us unable to distinguish between different function
templates that differ in only instantiation-dependent ways, for example

template<typename T> decltype(int(T())) f();
template<typename T> decltype(int(T(0))) f();

We'll need substantially better support for types that are
instantiation-dependent but not dependent before we can go ahead with
this change.

This reverts commit e3065ce238475ec202c707f4c58d90df171626ca.