Fix for dependent contexts in alias templates.

When we are parsing a type for an alias template, we are not entering
the context, so we can't look into dependent classes.  Make sure the
parser handles this correctly.

PR16904.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188510 91177308-0d34-0410-b5e6-96231b3b80d8