DebugInfo: Support type alias templates

We already got the type alias correct (though I've included a test case
here) since Clang represents that like any other typedef - but type
alias templates weren't being handled.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205691 91177308-0d34-0410-b5e6-96231b3b80d8