Minor bug fix for allowing an extension of const-qualified types in containers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@183481 91177308-0d34-0410-b5e6-96231b3b80d8
Test case for r183481.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@183522 91177308-0d34-0410-b5e6-96231b3b80d8