[index] Nested class declarations should be annotated with the

"specializationOf" relation if they pseudo-override a type in the base template

This commit fixes an issue where Xcode's renaming engine couldn't find the
reference to the second occurrence of "InnerClass" in this example:

template<typename T> struct Ts { template<typename U> struct InnerClass { }; };

template<> struct Ts<int> {
template<typename U> struct InnerClass; // This occurrence wasn't renamed
};

rdar://31884960

Differential Revision: https://reviews.llvm.org/D34392

llvm-svn: 305911