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
}

(This is the fixed version of r 311442, which was reverted in r311445.)

Patch by Ivan Donchevskii!

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

llvm-svn: 312780