[modules] Make NamespaceAliasDecl redeclarable, as it should be. This fixes
merging of namespace aliases across modules and improves source fidelity.
Incidentally also fixes PR20816.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217103 91177308-0d34-0410-b5e6-96231b3b80d8