PR26195: Set correct NestedNameSpecifierLoc for the dependent initializer

This commit fixes incorrect source positions of dependent c'tor initializers
like in the following code:

template<typename MyBase>
struct Derived: MyBase::InnerIterator
{

Derived() : MyBase::InnerIterator() {} /// This line is problematic: all positions point to InnerIterator and nothing points to MyBase
};

Patch by Serge Preis!

Differential Revision: https://reviews.llvm.org/D32439

llvm-svn: 306103