Fix templated type alias completion when using global completion cache

When we have enabled cache for global completions we did not have
diagnostics for Bar and could not complete Ba as in provided code
example.

template <typename T>
struct Foo { T member; };

template<typename T> using Bar = Foo<T>;

int main() {
    Ba
}

Patch by Ivan Donchevskii!

Differential Revision: https://reviews.llvm.org/D35355

llvm-svn: 311442