Partially roll back r166898; it exposed a bug in the standard.

The problem is as follows: C++11 has contexts which are not
potentially-evaluated, and yet in which we are required or encouraged to
perform constant evaluation. In such contexts, we are not permitted to
implicitly define special member functions for literal types, therefore
we cannot evalaute those constant expressions.

Punt on this in one more context for now by skipping checking constexpr
variable initializers if they occur in dependent contexts.

llvm-svn: 166956