[modules] Track the described template in an alias declaration that is the

[modules] Track the described template in an alias declaration that is the
pattern of an alias template declaration. Use this to merge alias templates
properly when they're members of class template specializations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216437 91177308-0d34-0410-b5e6-96231b3b80d8