Push alias-declarations and alias-template declarations into scope even if

Push alias-declarations and alias-template declarations into scope even if
they're redeclarations. This is necessary in order for name lookup to correctly
find the most recent declaration of the name (which affects default template
argument lookup and cross-module merging, among other things).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275612 91177308-0d34-0410-b5e6-96231b3b80d8